diff --git a/src/bloom.cpp b/src/bloom.cpp
index 1bfcbd406..26e366179 100644
--- a/src/bloom.cpp
+++ b/src/bloom.cpp
@@ -99,7 +99,7 @@ bool CBloomFilter::IsWithinSizeConstraints() const
     return vData.size() <= MAX_BLOOM_FILTER_SIZE && nHashFuncs <= MAX_HASH_FUNCS;
 }
 
-bool CBloomFilter::IsRelevantAndUpdate(const CTransaction& tx, const uint256& hash)
+bool CBloomFilter::IsRelevantAndUpdate(const CTransaction& tx)
 {
     bool fFound = false;
     // Match if the filter contains the hash of tx
@@ -108,6 +108,7 @@ bool CBloomFilter::IsRelevantAndUpdate(const CTransaction& tx, const uint256& ha
         return true;
     if (isEmpty)
         return false;
+    const uint256& hash = tx.GetHash();
     if (contains(hash))
         fFound = true;
 
diff --git a/src/bloom.h b/src/bloom.h
index 75e3f38c5..956bead87 100644
--- a/src/bloom.h
+++ b/src/bloom.h
@@ -83,7 +83,7 @@ public:
     bool IsWithinSizeConstraints() const;
 
     // Also adds any outputs which match the filter to the filter (to match their spending txes)
-    bool IsRelevantAndUpdate(const CTransaction& tx, const uint256& hash);
+    bool IsRelevantAndUpdate(const CTransaction& tx);
 
     // Checks for empty and full filters to avoid wasting cpu
     void UpdateEmptyFull();
diff --git a/src/chainparams.cpp b/src/chainparams.cpp
index 31eac62d4..afbae6fc5 100644
--- a/src/chainparams.cpp
+++ b/src/chainparams.cpp
@@ -127,7 +127,7 @@ public:
         //     CTxOut(nValue=50.00000000, scriptPubKey=0x5F1DF16B2B704C8A578D0B)
         //   vMerkleTree: 4a5e1e
         const char* pszTimestamp = "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks";
-        CTransaction txNew;
+        CMutableTransaction txNew;
         txNew.vin.resize(1);
         txNew.vout.resize(1);
         txNew.vin[0].scriptSig = CScript() << 486604799 << CScriptNum(4) << vector<unsigned char>((const unsigned char*)pszTimestamp, (const unsigned char*)pszTimestamp + strlen(pszTimestamp));
diff --git a/src/core.cpp b/src/core.cpp
index 6039986e6..6c5ee1c0f 100644
--- a/src/core.cpp
+++ b/src/core.cpp
@@ -91,11 +91,34 @@ std::string CFeeRate::ToString() const
     return result;
 }
 
-uint256 CTransaction::GetHash() const
+CMutableTransaction::CMutableTransaction() : nVersion(CTransaction::CURRENT_VERSION), nLockTime(0) {}
+CMutableTransaction::CMutableTransaction(const CTransaction& tx) : nVersion(tx.nVersion), vin(tx.vin), vout(tx.vout), nLockTime(tx.nLockTime) {}
+
+uint256 CMutableTransaction::GetHash() const
 {
     return SerializeHash(*this);
 }
 
+void CTransaction::UpdateHash() const
+{
+    *const_cast<uint256*>(&hash) = SerializeHash(*this);
+}
+
+CTransaction::CTransaction() : hash(0), nVersion(CTransaction::CURRENT_VERSION), vin(), vout(), nLockTime(0) { }
+
+CTransaction::CTransaction(const CMutableTransaction &tx) : nVersion(tx.nVersion), vin(tx.vin), vout(tx.vout), nLockTime(tx.nLockTime) {
+    UpdateHash();
+}
+
+CTransaction& CTransaction::operator=(const CTransaction &tx) {
+    *const_cast<int*>(&nVersion) = tx.nVersion;
+    *const_cast<std::vector<CTxIn>*>(&vin) = tx.vin;
+    *const_cast<std::vector<CTxOut>*>(&vout) = tx.vout;
+    *const_cast<unsigned int*>(&nLockTime) = tx.nLockTime;
+    *const_cast<uint256*>(&hash) = tx.hash;
+    return *this;
+}
+
 int64_t CTransaction::GetValueOut() const
 {
     int64_t nValueOut = 0;
diff --git a/src/core.h b/src/core.h
index 0e5912934..27fda9555 100644
--- a/src/core.h
+++ b/src/core.h
@@ -203,49 +203,59 @@ public:
 };
 
 
+struct CMutableTransaction;
+
 /** The basic transaction that is broadcasted on the network and contained in
  * blocks.  A transaction can contain multiple inputs and outputs.
  */
 class CTransaction
 {
+private:
+    /** Memory only. */
+    const uint256 hash;
+    void UpdateHash() const;
+
 public:
     static CFeeRate minTxFee;
     static CFeeRate minRelayTxFee;
     static const int CURRENT_VERSION=1;
-    int nVersion;
-    std::vector<CTxIn> vin;
-    std::vector<CTxOut> vout;
-    unsigned int nLockTime;
 
-    CTransaction()
-    {
-        SetNull();
-    }
+    // The local variables are made const to prevent unintended modification
+    // without updating the cached hash value. However, CTransaction is not
+    // actually immutable; deserialization and assignment are implemented,
+    // and bypass the constness. This is safe, as they update the entire
+    // structure, including the hash.
+    const int nVersion;
+    const std::vector<CTxIn> vin;
+    const std::vector<CTxOut> vout;
+    const unsigned int nLockTime;
 
-    IMPLEMENT_SERIALIZE
-    (
-        READWRITE(this->nVersion);
+    /** Construct a CTransaction that qualifies as IsNull() */
+    CTransaction();
+
+    /** Convert a CMutableTransaction into a CTransaction. */
+    CTransaction(const CMutableTransaction &tx);
+
+    CTransaction& operator=(const CTransaction& tx);
+
+    IMPLEMENT_SERIALIZE(
+        READWRITE(*const_cast<int*>(&this->nVersion));
         nVersion = this->nVersion;
-        READWRITE(vin);
-        READWRITE(vout);
-        READWRITE(nLockTime);
+        READWRITE(*const_cast<std::vector<CTxIn>*>(&vin));
+        READWRITE(*const_cast<std::vector<CTxOut>*>(&vout));
+        READWRITE(*const_cast<unsigned int*>(&nLockTime));
+        if (fRead)
+            UpdateHash();
     )
 
-    void SetNull()
-    {
-        nVersion = CTransaction::CURRENT_VERSION;
-        vin.clear();
-        vout.clear();
-        nLockTime = 0;
+    bool IsNull() const {
+        return vin.empty() && vout.empty();
     }
 
-    bool IsNull() const
-    {
-        return (vin.empty() && vout.empty());
+    const uint256& GetHash() const {
+        return hash;
     }
 
-    uint256 GetHash() const;
-
     // Return sum of txouts.
     int64_t GetValueOut() const;
     // GetValueIn() is a method on CCoinsViewCache, because
@@ -261,22 +271,43 @@ public:
 
     friend bool operator==(const CTransaction& a, const CTransaction& b)
     {
-        return (a.nVersion  == b.nVersion &&
-                a.vin       == b.vin &&
-                a.vout      == b.vout &&
-                a.nLockTime == b.nLockTime);
+        return a.hash == b.hash;
     }
 
     friend bool operator!=(const CTransaction& a, const CTransaction& b)
     {
-        return !(a == b);
+        return a.hash != b.hash;
     }
 
-
     std::string ToString() const;
     void print() const;
 };
 
+/** A mutable version of CTransaction. */
+struct CMutableTransaction
+{
+    int nVersion;
+    std::vector<CTxIn> vin;
+    std::vector<CTxOut> vout;
+    unsigned int nLockTime;
+
+    CMutableTransaction();
+    CMutableTransaction(const CTransaction& tx);
+
+    IMPLEMENT_SERIALIZE(
+        READWRITE(this->nVersion);
+        nVersion = this->nVersion;
+        READWRITE(vin);
+        READWRITE(vout);
+        READWRITE(nLockTime);
+    )
+
+    /** Compute the hash of this CMutableTransaction. This is computed on the
+     * fly, as opposed to GetHash() in CTransaction, which uses a cached result.
+     */
+    uint256 GetHash() const;
+};
+
 /** wrapper for CTxOut that provides a more compact serialization */
 class CTxOutCompressor
 {
@@ -465,12 +496,6 @@ public:
 
     uint256 BuildMerkleTree() const;
 
-    const uint256 &GetTxHash(unsigned int nIndex) const {
-        assert(vMerkleTree.size() > 0); // BuildMerkleTree must have been called first
-        assert(nIndex < vtx.size());
-        return vMerkleTree[nIndex];
-    }
-
     std::vector<uint256> GetMerkleBranch(int nIndex) const;
     static uint256 CheckMerkleBranch(uint256 hash, const std::vector<uint256>& vMerkleBranch, int nIndex);
     void print() const;
diff --git a/src/main.cpp b/src/main.cpp
index 006ec7cab..d3f04b95f 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -130,8 +130,8 @@ namespace {
 
 namespace {
 struct CMainSignals {
-    // Notifies listeners of updated transaction data (passing hash, transaction, and optionally the block it is found in.
-    boost::signals2::signal<void (const uint256 &, const CTransaction &, const CBlock *)> SyncTransaction;
+    // Notifies listeners of updated transaction data (transaction, and optionally the block it is found in.
+    boost::signals2::signal<void (const CTransaction &, const CBlock *)> SyncTransaction;
     // Notifies listeners of an erased transaction (currently disabled, requires transaction replacement).
     boost::signals2::signal<void (const uint256 &)> EraseTransaction;
     // Notifies listeners of an updated transaction without new data (for now: a coinbase potentially becoming visible).
@@ -146,7 +146,7 @@ struct CMainSignals {
 }
 
 void RegisterWallet(CWalletInterface* pwalletIn) {
-    g_signals.SyncTransaction.connect(boost::bind(&CWalletInterface::SyncTransaction, pwalletIn, _1, _2, _3));
+    g_signals.SyncTransaction.connect(boost::bind(&CWalletInterface::SyncTransaction, pwalletIn, _1, _2));
     g_signals.EraseTransaction.connect(boost::bind(&CWalletInterface::EraseFromWallet, pwalletIn, _1));
     g_signals.UpdatedTransaction.connect(boost::bind(&CWalletInterface::UpdatedTransaction, pwalletIn, _1));
     g_signals.SetBestChain.connect(boost::bind(&CWalletInterface::SetBestChain, pwalletIn, _1));
@@ -160,7 +160,7 @@ void UnregisterWallet(CWalletInterface* pwalletIn) {
     g_signals.SetBestChain.disconnect(boost::bind(&CWalletInterface::SetBestChain, pwalletIn, _1));
     g_signals.UpdatedTransaction.disconnect(boost::bind(&CWalletInterface::UpdatedTransaction, pwalletIn, _1));
     g_signals.EraseTransaction.disconnect(boost::bind(&CWalletInterface::EraseFromWallet, pwalletIn, _1));
-    g_signals.SyncTransaction.disconnect(boost::bind(&CWalletInterface::SyncTransaction, pwalletIn, _1, _2, _3));
+    g_signals.SyncTransaction.disconnect(boost::bind(&CWalletInterface::SyncTransaction, pwalletIn, _1, _2));
 }
 
 void UnregisterAllWallets() {
@@ -172,8 +172,8 @@ void UnregisterAllWallets() {
     g_signals.SyncTransaction.disconnect_all_slots();
 }
 
-void SyncWithWallets(const uint256 &hash, const CTransaction &tx, const CBlock *pblock) {
-    g_signals.SyncTransaction(hash, tx, pblock);
+void SyncWithWallets(const CTransaction &tx, const CBlock *pblock) {
+    g_signals.SyncTransaction(tx, pblock);
 }
 
 //////////////////////////////////////////////////////////////////////////////
@@ -952,7 +952,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
         pool.addUnchecked(hash, entry);
     }
 
-    g_signals.SyncTransaction(hash, tx, NULL);
+    g_signals.SyncTransaction(tx, NULL);
 
     return true;
 }
@@ -1479,7 +1479,7 @@ void UpdateTime(CBlockHeader& block, const CBlockIndex* pindexPrev)
 
 
 
-void UpdateCoins(const CTransaction& tx, CValidationState &state, CCoinsViewCache &inputs, CTxUndo &txundo, int nHeight, const uint256 &txhash)
+void UpdateCoins(const CTransaction& tx, CValidationState &state, CCoinsViewCache &inputs, CTxUndo &txundo, int nHeight)
 {
     bool ret;
     // mark inputs spent
@@ -1494,7 +1494,7 @@ void UpdateCoins(const CTransaction& tx, CValidationState &state, CCoinsViewCach
     }
 
     // add outputs
-    ret = inputs.SetCoins(txhash, CCoins(tx, nHeight));
+    ret = inputs.SetCoins(tx.GetHash(), CCoins(tx, nHeight));
     assert(ret);
 }
 
@@ -1767,8 +1767,8 @@ bool ConnectBlock(CBlock& block, CValidationState& state, CBlockIndex* pindex, C
                           !((pindex->nHeight==91842 && pindex->GetBlockHash() == uint256("0x00000000000a4d0a398161ffc163c503763b1f4360639393e0e4c8e300e0caec")) ||
                            (pindex->nHeight==91880 && pindex->GetBlockHash() == uint256("0x00000000000743f190a18c5577a3c2d2a1f610ae9601ac046a38084ccb7cd721")));
     if (fEnforceBIP30) {
-        for (unsigned int i = 0; i < block.vtx.size(); i++) {
-            uint256 hash = block.GetTxHash(i);
+        BOOST_FOREACH(const CTransaction& tx, block.vtx) {
+            const uint256& hash = tx.GetHash();
             if (view.HaveCoins(hash) && !view.GetCoins(hash).IsPruned())
                 return state.DoS(100, error("ConnectBlock() : tried to overwrite transaction"),
                                  REJECT_INVALID, "bad-txns-BIP30");
@@ -1829,11 +1829,11 @@ bool ConnectBlock(CBlock& block, CValidationState& state, CBlockIndex* pindex, C
         }
 
         CTxUndo txundo;
-        UpdateCoins(tx, state, view, txundo, pindex->nHeight, block.GetTxHash(i));
+        UpdateCoins(tx, state, view, txundo, pindex->nHeight);
         if (!tx.IsCoinBase())
             blockundo.vtxundo.push_back(txundo);
 
-        vPos.push_back(std::make_pair(block.GetTxHash(i), pos));
+        vPos.push_back(std::make_pair(tx.GetHash(), pos));
         pos.nTxOffset += ::GetSerializeSize(tx, SER_DISK, CLIENT_VERSION);
     }
     int64_t nTime = GetTimeMicros() - nStart;
@@ -1892,13 +1892,13 @@ bool ConnectBlock(CBlock& block, CValidationState& state, CBlockIndex* pindex, C
     assert(ret);
 
     // Watch for transactions paying to me
-    for (unsigned int i = 0; i < block.vtx.size(); i++)
-        g_signals.SyncTransaction(block.GetTxHash(i), block.vtx[i], &block);
+    BOOST_FOREACH(const CTransaction& tx, block.vtx)
+        g_signals.SyncTransaction(tx, &block);
 
     // Watch for changes to the previous coinbase transaction.
     static uint256 hashPrevBestCoinBase;
     g_signals.UpdatedTransaction(hashPrevBestCoinBase);
-    hashPrevBestCoinBase = block.GetTxHash(0);
+    hashPrevBestCoinBase = block.vtx[0].GetHash();
 
     return true;
 }
@@ -1996,7 +1996,7 @@ bool static DisconnectTip(CValidationState &state) {
     // Let wallets know transactions went from 1-confirmed to
     // 0-confirmed or conflicted:
     BOOST_FOREACH(const CTransaction &tx, block.vtx) {
-        SyncWithWallets(tx.GetHash(), tx, NULL);
+        SyncWithWallets(tx, NULL);
     }
     return true;
 }
@@ -2036,11 +2036,11 @@ bool static ConnectTip(CValidationState &state, CBlockIndex *pindexNew) {
     // Tell wallet about transactions that went from mempool
     // to conflicted:
     BOOST_FOREACH(const CTransaction &tx, txConflicted) {
-        SyncWithWallets(tx.GetHash(), tx, NULL);
+        SyncWithWallets(tx, NULL);
     }
     // ... and about transactions that got confirmed:
     BOOST_FOREACH(const CTransaction &tx, block.vtx) {
-        SyncWithWallets(tx.GetHash(), tx, &block);
+        SyncWithWallets(tx, &block);
     }
     return true;
 }
@@ -2381,16 +2381,11 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
         if (!CheckTransaction(tx, state))
             return error("CheckBlock() : CheckTransaction failed");
 
-    // Build the merkle tree already. We need it anyway later, and it makes the
-    // block cache the transaction hashes, which means they don't need to be
-    // recalculated many times during this block's validation.
-    block.BuildMerkleTree();
-
     // Check for duplicate txids. This is caught by ConnectInputs(),
     // but catching it earlier avoids a potential DoS attack:
     set<uint256> uniqueTx;
-    for (unsigned int i = 0; i < block.vtx.size(); i++) {
-        uniqueTx.insert(block.GetTxHash(i));
+    BOOST_FOREACH(const CTransaction &tx, block.vtx) {
+        uniqueTx.insert(tx.GetHash());
     }
     if (uniqueTx.size() != block.vtx.size())
         return state.DoS(100, error("CheckBlock() : duplicate transaction"),
@@ -2406,7 +2401,7 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
                          REJECT_INVALID, "bad-blk-sigops", true);
 
     // Check merkle root
-    if (fCheckMerkleRoot && block.hashMerkleRoot != block.vMerkleTree.back())
+    if (fCheckMerkleRoot && block.hashMerkleRoot != block.BuildMerkleTree())
         return state.DoS(100, error("CheckBlock() : hashMerkleRoot mismatch"),
                          REJECT_INVALID, "bad-txnmrklroot", true);
 
@@ -2682,8 +2677,8 @@ CMerkleBlock::CMerkleBlock(const CBlock& block, CBloomFilter& filter)
 
     for (unsigned int i = 0; i < block.vtx.size(); i++)
     {
-        uint256 hash = block.vtx[i].GetHash();
-        if (filter.IsRelevantAndUpdate(block.vtx[i], hash))
+        const uint256& hash = block.vtx[i].GetHash();
+        if (filter.IsRelevantAndUpdate(block.vtx[i]))
         {
             vMatch.push_back(true);
             vMatchedTxn.push_back(make_pair(i, hash));
@@ -3832,7 +3827,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
         if (AcceptToMemoryPool(mempool, state, tx, true, &fMissingInputs))
         {
             mempool.check(pcoinsTip);
-            RelayTransaction(tx, inv.hash);
+            RelayTransaction(tx);
             mapAlreadyAskedFor.erase(inv);
             vWorkQueue.push_back(inv.hash);
             vEraseQueue.push_back(inv.hash);
@@ -3862,7 +3857,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
                     if (AcceptToMemoryPool(mempool, stateDummy, orphanTx, true, &fMissingInputs2))
                     {
                         LogPrint("mempool", "   accepted orphan tx %s\n", orphanHash.ToString());
-                        RelayTransaction(orphanTx, orphanHash);
+                        RelayTransaction(orphanTx);
                         mapAlreadyAskedFor.erase(CInv(MSG_TX, orphanHash));
                         vWorkQueue.push_back(orphanHash);
                         vEraseQueue.push_back(orphanHash);
@@ -3947,7 +3942,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
             CTransaction tx;
             bool fInMemPool = mempool.lookup(hash, tx);
             if (!fInMemPool) continue; // another thread removed since queryHashes, maybe...
-            if ((pfrom->pfilter && pfrom->pfilter->IsRelevantAndUpdate(tx, hash)) ||
+            if ((pfrom->pfilter && pfrom->pfilter->IsRelevantAndUpdate(tx)) ||
                (!pfrom->pfilter))
                 vInv.push_back(inv);
             if (vInv.size() == MAX_INV_SZ) {
diff --git a/src/main.h b/src/main.h
index 5a0aedcde..7071f0094 100644
--- a/src/main.h
+++ b/src/main.h
@@ -113,7 +113,7 @@ void UnregisterWallet(CWalletInterface* pwalletIn);
 /** Unregister all wallets from core */
 void UnregisterAllWallets();
 /** Push an updated transaction to all registered wallets */
-void SyncWithWallets(const uint256 &hash, const CTransaction& tx, const CBlock* pblock = NULL);
+void SyncWithWallets(const CTransaction& tx, const CBlock* pblock = NULL);
 
 /** Register with a network node to receive its signals */
 void RegisterNodeSignals(CNodeSignals& nodeSignals);
@@ -294,7 +294,7 @@ bool CheckInputs(const CTransaction& tx, CValidationState &state, CCoinsViewCach
                  std::vector<CScriptCheck> *pvChecks = NULL);
 
 // Apply the effects of this transaction on the UTXO set represented by view
-void UpdateCoins(const CTransaction& tx, CValidationState &state, CCoinsViewCache &inputs, CTxUndo &txundo, int nHeight, const uint256 &txhash);
+void UpdateCoins(const CTransaction& tx, CValidationState &state, CCoinsViewCache &inputs, CTxUndo &txundo, int nHeight);
 
 // Context-independent validity checks
 bool CheckTransaction(const CTransaction& tx, CValidationState& state);
@@ -1129,7 +1129,7 @@ public:
 
 class CWalletInterface {
 protected:
-    virtual void SyncTransaction(const uint256 &hash, const CTransaction &tx, const CBlock *pblock) =0;
+    virtual void SyncTransaction(const CTransaction &tx, const CBlock *pblock) =0;
     virtual void EraseFromWallet(const uint256 &hash) =0;
     virtual void SetBestChain(const CBlockLocator &locator) =0;
     virtual void UpdatedTransaction(const uint256 &hash) =0;
diff --git a/src/miner.cpp b/src/miner.cpp
index 7efca7cff..63ce12506 100644
--- a/src/miner.cpp
+++ b/src/miner.cpp
@@ -86,14 +86,14 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
     CBlock *pblock = &pblocktemplate->block; // pointer for convenience
 
     // Create coinbase tx
-    CTransaction txNew;
+    CMutableTransaction txNew;
     txNew.vin.resize(1);
     txNew.vin[0].prevout.SetNull();
     txNew.vout.resize(1);
     txNew.vout[0].scriptPubKey = scriptPubKeyIn;
 
-    // Add our coinbase tx as first transaction
-    pblock->vtx.push_back(txNew);
+    // Add dummy coinbase tx as first transaction
+    pblock->vtx.push_back(CTransaction());
     pblocktemplate->vTxFees.push_back(-1); // updated at end
     pblocktemplate->vTxSigOps.push_back(-1); // updated at end
 
@@ -254,8 +254,8 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
                 continue;
 
             CTxUndo txundo;
-            uint256 hash = tx.GetHash();
-            UpdateCoins(tx, state, view, txundo, pindexPrev->nHeight+1, hash);
+            const uint256& hash = tx.GetHash();
+            UpdateCoins(tx, state, view, txundo, pindexPrev->nHeight+1);
 
             // Added
             pblock->vtx.push_back(tx);
@@ -294,7 +294,10 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         nLastBlockSize = nBlockSize;
         LogPrintf("CreateNewBlock(): total size %u\n", nBlockSize);
 
-        pblock->vtx[0].vout[0].nValue = GetBlockValue(pindexPrev->nHeight+1, nFees);
+        // Compute final coinbase transaction.
+        txNew.vout[0].nValue = GetBlockValue(pindexPrev->nHeight+1, nFees);
+        txNew.vin[0].scriptSig = CScript() << OP_0 << OP_0;
+        pblock->vtx[0] = txNew;
         pblocktemplate->vTxFees[0] = -nFees;
 
         // Fill in header
@@ -302,7 +305,6 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         UpdateTime(*pblock, pindexPrev);
         pblock->nBits          = GetNextWorkRequired(pindexPrev, pblock);
         pblock->nNonce         = 0;
-        pblock->vtx[0].vin[0].scriptSig = CScript() << OP_0 << OP_0;
         pblocktemplate->vTxSigOps[0] = GetLegacySigOpCount(pblock->vtx[0]);
 
         CBlockIndex indexDummy(*pblock);
@@ -328,9 +330,11 @@ void IncrementExtraNonce(CBlock* pblock, CBlockIndex* pindexPrev, unsigned int&
     }
     ++nExtraNonce;
     unsigned int nHeight = pindexPrev->nHeight+1; // Height first in coinbase required for block.version=2
-    pblock->vtx[0].vin[0].scriptSig = (CScript() << nHeight << CScriptNum(nExtraNonce)) + COINBASE_FLAGS;
-    assert(pblock->vtx[0].vin[0].scriptSig.size() <= 100);
+    CMutableTransaction txCoinbase(pblock->vtx[0]);
+    txCoinbase.vin[0].scriptSig = (CScript() << nHeight << CScriptNum(nExtraNonce)) + COINBASE_FLAGS;
+    assert(txCoinbase.vin[0].scriptSig.size() <= 100);
 
+    pblock->vtx[0] = txCoinbase;
     pblock->hashMerkleRoot = pblock->BuildMerkleTree();
 }
 
diff --git a/src/net.cpp b/src/net.cpp
index 707f58f8a..22077777c 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -1812,17 +1812,17 @@ instance_of_cnetcleanup;
 
 
 
-void RelayTransaction(const CTransaction& tx, const uint256& hash)
+void RelayTransaction(const CTransaction& tx)
 {
     CDataStream ss(SER_NETWORK, PROTOCOL_VERSION);
     ss.reserve(10000);
     ss << tx;
-    RelayTransaction(tx, hash, ss);
+    RelayTransaction(tx, ss);
 }
 
-void RelayTransaction(const CTransaction& tx, const uint256& hash, const CDataStream& ss)
+void RelayTransaction(const CTransaction& tx, const CDataStream& ss)
 {
-    CInv inv(MSG_TX, hash);
+    CInv inv(MSG_TX, tx.GetHash());
     {
         LOCK(cs_mapRelay);
         // Expire old relay messages
@@ -1844,7 +1844,7 @@ void RelayTransaction(const CTransaction& tx, const uint256& hash, const CDataSt
         LOCK(pnode->cs_filter);
         if (pnode->pfilter)
         {
-            if (pnode->pfilter->IsRelevantAndUpdate(tx, hash))
+            if (pnode->pfilter->IsRelevantAndUpdate(tx))
                 pnode->PushInventory(inv);
         } else
             pnode->PushInventory(inv);
diff --git a/src/net.h b/src/net.h
index 14d578a72..eec92f340 100644
--- a/src/net.h
+++ b/src/net.h
@@ -726,8 +726,8 @@ public:
 
 
 class CTransaction;
-void RelayTransaction(const CTransaction& tx, const uint256& hash);
-void RelayTransaction(const CTransaction& tx, const uint256& hash, const CDataStream& ss);
+void RelayTransaction(const CTransaction& tx);
+void RelayTransaction(const CTransaction& tx, const CDataStream& ss);
 
 /** Access to the (IP) address database (peers.dat) */
 class CAddrDB
diff --git a/src/qt/coincontroldialog.cpp b/src/qt/coincontroldialog.cpp
index 42d6da7d3..52bdf9673 100644
--- a/src/qt/coincontroldialog.cpp
+++ b/src/qt/coincontroldialog.cpp
@@ -440,7 +440,7 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
     // nPayAmount
     qint64 nPayAmount = 0;
     bool fDust = false;
-    CTransaction txDummy;
+    CMutableTransaction txDummy;
     foreach(const qint64 &amount, CoinControlDialog::payAmounts)
     {
         nPayAmount += amount;
diff --git a/src/rpcrawtransaction.cpp b/src/rpcrawtransaction.cpp
index dee7daeb2..9771f8e68 100644
--- a/src/rpcrawtransaction.cpp
+++ b/src/rpcrawtransaction.cpp
@@ -349,7 +349,7 @@ Value createrawtransaction(const Array& params, bool fHelp)
     Array inputs = params[0].get_array();
     Object sendTo = params[1].get_obj();
 
-    CTransaction rawTx;
+    CMutableTransaction rawTx;
 
     BOOST_FOREACH(const Value& input, inputs)
     {
@@ -554,11 +554,11 @@ Value signrawtransaction(const Array& params, bool fHelp)
 
     vector<unsigned char> txData(ParseHexV(params[0], "argument 1"));
     CDataStream ssData(txData, SER_NETWORK, PROTOCOL_VERSION);
-    vector<CTransaction> txVariants;
+    vector<CMutableTransaction> txVariants;
     while (!ssData.empty())
     {
         try {
-            CTransaction tx;
+            CMutableTransaction tx;
             ssData >> tx;
             txVariants.push_back(tx);
         }
@@ -572,7 +572,7 @@ Value signrawtransaction(const Array& params, bool fHelp)
 
     // mergedTx will end up with all the signatures; it
     // starts as a clone of the rawtx:
-    CTransaction mergedTx(txVariants[0]);
+    CMutableTransaction mergedTx(txVariants[0]);
     bool fComplete = true;
 
     // Fetch previous transactions (inputs):
@@ -713,7 +713,7 @@ Value signrawtransaction(const Array& params, bool fHelp)
             SignSignature(keystore, prevPubKey, mergedTx, i, nHashType);
 
         // ... and merge in other signatures:
-        BOOST_FOREACH(const CTransaction& txv, txVariants)
+        BOOST_FOREACH(const CMutableTransaction& txv, txVariants)
         {
             txin.scriptSig = CombineSignatures(prevPubKey, mergedTx, i, txin.scriptSig, txv.vin[i].scriptSig);
         }
@@ -770,7 +770,7 @@ Value sendrawtransaction(const Array& params, bool fHelp)
     catch (std::exception &e) {
         throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
     }
-    uint256 hashTx = tx.GetHash();
+    const uint256 &hashTx = tx.GetHash();
 
     CCoinsViewCache &view = *pcoinsTip;
     CCoins existingCoins;
@@ -780,7 +780,7 @@ Value sendrawtransaction(const Array& params, bool fHelp)
         // push to local node and sync with wallets
         CValidationState state;
         if (AcceptToMemoryPool(mempool, state, tx, false, NULL, !fOverrideFees))
-            SyncWithWallets(hashTx, tx, NULL);
+            SyncWithWallets(tx, NULL);
         else {
             if(state.IsInvalid())
                 throw JSONRPCError(RPC_TRANSACTION_REJECTED, strprintf("%i: %s", state.GetRejectCode(), state.GetRejectReason()));
@@ -790,7 +790,7 @@ Value sendrawtransaction(const Array& params, bool fHelp)
     } else if (fHaveChain) {
         throw JSONRPCError(RPC_TRANSACTION_ALREADY_IN_CHAIN, "transaction already in block chain");
     }
-    RelayTransaction(tx, hashTx);
+    RelayTransaction(tx);
 
     return hashTx.GetHex();
 }
diff --git a/src/script.cpp b/src/script.cpp
index b383a00a6..c83d26885 100644
--- a/src/script.cpp
+++ b/src/script.cpp
@@ -1636,7 +1636,7 @@ bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, const C
 }
 
 
-bool SignSignature(const CKeyStore &keystore, const CScript& fromPubKey, CTransaction& txTo, unsigned int nIn, int nHashType)
+bool SignSignature(const CKeyStore &keystore, const CScript& fromPubKey, CMutableTransaction& txTo, unsigned int nIn, int nHashType)
 {
     assert(nIn < txTo.vin.size());
     CTxIn& txin = txTo.vin[nIn];
@@ -1671,7 +1671,7 @@ bool SignSignature(const CKeyStore &keystore, const CScript& fromPubKey, CTransa
     return VerifyScript(txin.scriptSig, fromPubKey, txTo, nIn, STANDARD_SCRIPT_VERIFY_FLAGS, 0);
 }
 
-bool SignSignature(const CKeyStore &keystore, const CTransaction& txFrom, CTransaction& txTo, unsigned int nIn, int nHashType)
+bool SignSignature(const CKeyStore &keystore, const CTransaction& txFrom, CMutableTransaction& txTo, unsigned int nIn, int nHashType)
 {
     assert(nIn < txTo.vin.size());
     CTxIn& txin = txTo.vin[nIn];
@@ -1689,7 +1689,7 @@ static CScript PushAll(const vector<valtype>& values)
     return result;
 }
 
-static CScript CombineMultisig(CScript scriptPubKey, const CTransaction& txTo, unsigned int nIn,
+static CScript CombineMultisig(CScript scriptPubKey, const CMutableTransaction& txTo, unsigned int nIn,
                                const vector<valtype>& vSolutions,
                                vector<valtype>& sigs1, vector<valtype>& sigs2)
 {
diff --git a/src/script.h b/src/script.h
index a282e7dc0..bd6574627 100644
--- a/src/script.h
+++ b/src/script.h
@@ -20,6 +20,7 @@
 class CCoins;
 class CKeyStore;
 class CTransaction;
+class CMutableTransaction;
 
 static const unsigned int MAX_SCRIPT_ELEMENT_SIZE = 520; // bytes
 static const unsigned int MAX_OP_RETURN_RELAY = 40;      // bytes
@@ -805,8 +806,8 @@ bool IsMine(const CKeyStore& keystore, const CTxDestination &dest);
 void ExtractAffectedKeys(const CKeyStore &keystore, const CScript& scriptPubKey, std::vector<CKeyID> &vKeys);
 bool ExtractDestination(const CScript& scriptPubKey, CTxDestination& addressRet);
 bool ExtractDestinations(const CScript& scriptPubKey, txnouttype& typeRet, std::vector<CTxDestination>& addressRet, int& nRequiredRet);
-bool SignSignature(const CKeyStore& keystore, const CScript& fromPubKey, CTransaction& txTo, unsigned int nIn, int nHashType=SIGHASH_ALL);
-bool SignSignature(const CKeyStore& keystore, const CTransaction& txFrom, CTransaction& txTo, unsigned int nIn, int nHashType=SIGHASH_ALL);
+bool SignSignature(const CKeyStore& keystore, const CScript& fromPubKey, CMutableTransaction& txTo, unsigned int nIn, int nHashType=SIGHASH_ALL);
+bool SignSignature(const CKeyStore& keystore, const CTransaction& txFrom, CMutableTransaction& txTo, unsigned int nIn, int nHashType=SIGHASH_ALL);
 bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, const CTransaction& txTo, unsigned int nIn, unsigned int flags, int nHashType);
 
 // Given two sets of signatures for scriptPubKey, possibly with OP_0 placeholders,
diff --git a/src/test/DoS_tests.cpp b/src/test/DoS_tests.cpp
index fb06fb343..3a4584441 100644
--- a/src/test/DoS_tests.cpp
+++ b/src/test/DoS_tests.cpp
@@ -167,7 +167,7 @@ BOOST_AUTO_TEST_CASE(DoS_mapOrphans)
     // 50 orphan transactions:
     for (int i = 0; i < 50; i++)
     {
-        CTransaction tx;
+        CMutableTransaction tx;
         tx.vin.resize(1);
         tx.vin[0].prevout.n = 0;
         tx.vin[0].prevout.hash = GetRandHash();
@@ -184,7 +184,7 @@ BOOST_AUTO_TEST_CASE(DoS_mapOrphans)
     {
         CTransaction txPrev = RandomOrphan();
 
-        CTransaction tx;
+        CMutableTransaction tx;
         tx.vin.resize(1);
         tx.vin[0].prevout.n = 0;
         tx.vin[0].prevout.hash = txPrev.GetHash();
@@ -201,7 +201,7 @@ BOOST_AUTO_TEST_CASE(DoS_mapOrphans)
     {
         CTransaction txPrev = RandomOrphan();
 
-        CTransaction tx;
+        CMutableTransaction tx;
         tx.vout.resize(1);
         tx.vout[0].nValue = 1*CENT;
         tx.vout[0].scriptPubKey.SetDestination(key.GetPubKey().GetID());
@@ -242,10 +242,10 @@ BOOST_AUTO_TEST_CASE(DoS_checkSig)
 
     // 100 orphan transactions:
     static const int NPREV=100;
-    CTransaction orphans[NPREV];
+    CMutableTransaction orphans[NPREV];
     for (int i = 0; i < NPREV; i++)
     {
-        CTransaction& tx = orphans[i];
+        CMutableTransaction& tx = orphans[i];
         tx.vin.resize(1);
         tx.vin[0].prevout.n = 0;
         tx.vin[0].prevout.hash = GetRandHash();
@@ -258,7 +258,7 @@ BOOST_AUTO_TEST_CASE(DoS_checkSig)
     }
 
     // Create a transaction that depends on orphans:
-    CTransaction tx;
+    CMutableTransaction tx;
     tx.vout.resize(1);
     tx.vout[0].nValue = 1*CENT;
     tx.vout[0].scriptPubKey.SetDestination(key.GetPubKey().GetID());
diff --git a/src/test/accounting_tests.cpp b/src/test/accounting_tests.cpp
index e2a75da34..4bee0f6b6 100644
--- a/src/test/accounting_tests.cpp
+++ b/src/test/accounting_tests.cpp
@@ -83,13 +83,21 @@ BOOST_AUTO_TEST_CASE(acc_orderupgrade)
 
 
     wtx.mapValue["comment"] = "y";
-    --wtx.nLockTime;  // Just to change the hash :)
+    {
+        CMutableTransaction tx(wtx);
+        --tx.nLockTime;  // Just to change the hash :)
+        *static_cast<CTransaction*>(&wtx) = CTransaction(tx);
+    }
     pwalletMain->AddToWallet(wtx);
     vpwtx.push_back(&pwalletMain->mapWallet[wtx.GetHash()]);
     vpwtx[1]->nTimeReceived = (unsigned int)1333333336;
 
     wtx.mapValue["comment"] = "x";
-    --wtx.nLockTime;  // Just to change the hash :)
+    {
+        CMutableTransaction tx(wtx);
+        --tx.nLockTime;  // Just to change the hash :)
+        *static_cast<CTransaction*>(&wtx) = CTransaction(tx);
+    }
     pwalletMain->AddToWallet(wtx);
     vpwtx.push_back(&pwalletMain->mapWallet[wtx.GetHash()]);
     vpwtx[2]->nTimeReceived = (unsigned int)1333333329;
diff --git a/src/test/bloom_tests.cpp b/src/test/bloom_tests.cpp
index abedd3093..b56d998be 100644
--- a/src/test/bloom_tests.cpp
+++ b/src/test/bloom_tests.cpp
@@ -118,33 +118,33 @@ BOOST_AUTO_TEST_CASE(bloom_match)
 
     CBloomFilter filter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(uint256("0xb4749f017444b051c44dfd2720e88f314ff94f3dd6d56d40ef65854fcd7fff6b"));
-    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter didn't match tx hash");
+    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx), "Simple Bloom filter didn't match tx hash");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     // byte-reversed tx hash
     filter.insert(ParseHex("6bff7fcd4f8565ef406dd5d63d4ff94f318fe82027fd4dc451b04474019f74b4"));
-    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter didn't match manually serialized tx hash");
+    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx), "Simple Bloom filter didn't match manually serialized tx hash");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(ParseHex("30450220070aca44506c5cef3a16ed519d7c3c39f8aab192c4e1c90d065f37b8a4af6141022100a8e160b856c2d43d27d8fba71e5aef6405b8643ac4cb7cb3c462aced7f14711a01"));
-    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter didn't match input signature");
+    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx), "Simple Bloom filter didn't match input signature");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(ParseHex("046d11fee51b0e60666d5049a9101a72741df480b96ee26488a4d3466b95c9a40ac5eeef87e10a5cd336c19a84565f80fa6c547957b7700ff4dfbdefe76036c339"));
-    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter didn't match input pub key");
+    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx), "Simple Bloom filter didn't match input pub key");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(ParseHex("04943fdd508053c75000106d3bc6e2754dbcff19"));
-    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter didn't match output address");
-    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(spendingTx, spendingTx.GetHash()), "Simple Bloom filter didn't add output");
+    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx), "Simple Bloom filter didn't match output address");
+    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(spendingTx), "Simple Bloom filter didn't add output");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(ParseHex("a266436d2965547608b9e15d9032a7b9d64fa431"));
-    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter didn't match output address");
+    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx), "Simple Bloom filter didn't match output address");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(COutPoint(uint256("0x90c122d70786e899529d71dbeba91ba216982fb6ba58f3bdaab65e73b7e9260b"), 0));
-    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter didn't match COutPoint");
+    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx), "Simple Bloom filter didn't match COutPoint");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     COutPoint prevOutPoint(uint256("0x90c122d70786e899529d71dbeba91ba216982fb6ba58f3bdaab65e73b7e9260b"), 0);
@@ -154,23 +154,23 @@ BOOST_AUTO_TEST_CASE(bloom_match)
         memcpy(&data[32], &prevOutPoint.n, sizeof(unsigned int));
         filter.insert(data);
     }
-    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter didn't match manually serialized COutPoint");
+    BOOST_CHECK_MESSAGE(filter.IsRelevantAndUpdate(tx), "Simple Bloom filter didn't match manually serialized COutPoint");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(uint256("00000009e784f32f62ef849763d4f45b98e07ba658647343b915ff832b110436"));
-    BOOST_CHECK_MESSAGE(!filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter matched random tx hash");
+    BOOST_CHECK_MESSAGE(!filter.IsRelevantAndUpdate(tx), "Simple Bloom filter matched random tx hash");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(ParseHex("0000006d2965547608b9e15d9032a7b9d64fa431"));
-    BOOST_CHECK_MESSAGE(!filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter matched random address");
+    BOOST_CHECK_MESSAGE(!filter.IsRelevantAndUpdate(tx), "Simple Bloom filter matched random address");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(COutPoint(uint256("0x90c122d70786e899529d71dbeba91ba216982fb6ba58f3bdaab65e73b7e9260b"), 1));
-    BOOST_CHECK_MESSAGE(!filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter matched COutPoint for an output we didn't care about");
+    BOOST_CHECK_MESSAGE(!filter.IsRelevantAndUpdate(tx), "Simple Bloom filter matched COutPoint for an output we didn't care about");
 
     filter = CBloomFilter(10, 0.000001, 0, BLOOM_UPDATE_ALL);
     filter.insert(COutPoint(uint256("0x000000d70786e899529d71dbeba91ba216982fb6ba58f3bdaab65e73b7e9260b"), 0));
-    BOOST_CHECK_MESSAGE(!filter.IsRelevantAndUpdate(tx, tx.GetHash()), "Simple Bloom filter matched COutPoint for an output we didn't care about");
+    BOOST_CHECK_MESSAGE(!filter.IsRelevantAndUpdate(tx), "Simple Bloom filter matched COutPoint for an output we didn't care about");
 }
 
 BOOST_AUTO_TEST_CASE(merkle_block_1)
diff --git a/src/test/miner_tests.cpp b/src/test/miner_tests.cpp
index bff6de41a..47977cf29 100644
--- a/src/test/miner_tests.cpp
+++ b/src/test/miner_tests.cpp
@@ -51,7 +51,7 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
 {
     CScript scriptPubKey = CScript() << ParseHex("04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5f") << OP_CHECKSIG;
     CBlockTemplate *pblocktemplate;
-    CTransaction tx,tx2;
+    CMutableTransaction tx,tx2;
     CScript script;
     uint256 hash;
 
@@ -68,10 +68,12 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
         CBlock *pblock = &pblocktemplate->block; // pointer for convenience
         pblock->nVersion = 1;
         pblock->nTime = chainActive.Tip()->GetMedianTimePast()+1;
-        pblock->vtx[0].vin[0].scriptSig = CScript();
-        pblock->vtx[0].vin[0].scriptSig.push_back(blockinfo[i].extranonce);
-        pblock->vtx[0].vin[0].scriptSig.push_back(chainActive.Height());
-        pblock->vtx[0].vout[0].scriptPubKey = CScript();
+        CMutableTransaction txCoinbase(pblock->vtx[0]);
+        txCoinbase.vin[0].scriptSig = CScript();
+        txCoinbase.vin[0].scriptSig.push_back(blockinfo[i].extranonce);
+        txCoinbase.vin[0].scriptSig.push_back(chainActive.Height());
+        txCoinbase.vout[0].scriptPubKey = CScript();
+        pblock->vtx[0] = CTransaction(txCoinbase);
         if (txFirst.size() < 2)
             txFirst.push_back(new CTransaction(pblock->vtx[0]));
         pblock->hashMerkleRoot = pblock->BuildMerkleTree();
diff --git a/src/test/multisig_tests.cpp b/src/test/multisig_tests.cpp
index 3775abd63..452cf084a 100644
--- a/src/test/multisig_tests.cpp
+++ b/src/test/multisig_tests.cpp
@@ -55,13 +55,13 @@ BOOST_AUTO_TEST_CASE(multisig_verify)
     CScript escrow;
     escrow << OP_2 << key[0].GetPubKey() << key[1].GetPubKey() << key[2].GetPubKey() << OP_3 << OP_CHECKMULTISIG;
 
-    CTransaction txFrom;  // Funding transaction
+    CMutableTransaction txFrom;  // Funding transaction
     txFrom.vout.resize(3);
     txFrom.vout[0].scriptPubKey = a_and_b;
     txFrom.vout[1].scriptPubKey = a_or_b;
     txFrom.vout[2].scriptPubKey = escrow;
 
-    CTransaction txTo[3]; // Spending transaction
+    CMutableTransaction txTo[3]; // Spending transaction
     for (int i = 0; i < 3; i++)
     {
         txTo[i].vin.resize(1);
@@ -270,13 +270,13 @@ BOOST_AUTO_TEST_CASE(multisig_Sign)
     CScript escrow;
     escrow << OP_2 << key[0].GetPubKey() << key[1].GetPubKey() << key[2].GetPubKey() << OP_3 << OP_CHECKMULTISIG;
 
-    CTransaction txFrom;  // Funding transaction
+    CMutableTransaction txFrom;  // Funding transaction
     txFrom.vout.resize(3);
     txFrom.vout[0].scriptPubKey = a_and_b;
     txFrom.vout[1].scriptPubKey = a_or_b;
     txFrom.vout[2].scriptPubKey = escrow;
 
-    CTransaction txTo[3]; // Spending transaction
+    CMutableTransaction txTo[3]; // Spending transaction
     for (int i = 0; i < 3; i++)
     {
         txTo[i].vin.resize(1);
diff --git a/src/test/pmt_tests.cpp b/src/test/pmt_tests.cpp
index 7d7e6681d..9dce4daac 100644
--- a/src/test/pmt_tests.cpp
+++ b/src/test/pmt_tests.cpp
@@ -36,9 +36,9 @@ BOOST_AUTO_TEST_CASE(pmt_test1)
         // build a block with some dummy transactions
         CBlock block;
         for (unsigned int j=0; j<nTx; j++) {
-            CTransaction tx;
+            CMutableTransaction tx;
             tx.nLockTime = rand(); // actual transaction data doesn't matter; just make the nLockTime's unique
-            block.vtx.push_back(tx);
+            block.vtx.push_back(CTransaction(tx));
         }
 
         // calculate actual merkle root and height
diff --git a/src/test/script_P2SH_tests.cpp b/src/test/script_P2SH_tests.cpp
index 9b1290e0e..a75593a8b 100644
--- a/src/test/script_P2SH_tests.cpp
+++ b/src/test/script_P2SH_tests.cpp
@@ -30,11 +30,11 @@ static bool
 Verify(const CScript& scriptSig, const CScript& scriptPubKey, bool fStrict)
 {
     // Create dummy to/from transactions:
-    CTransaction txFrom;
+    CMutableTransaction txFrom;
     txFrom.vout.resize(1);
     txFrom.vout[0].scriptPubKey = scriptPubKey;
 
-    CTransaction txTo;
+    CMutableTransaction txTo;
     txTo.vin.resize(1);
     txTo.vout.resize(1);
     txTo.vin[0].prevout.n = 0;
@@ -78,7 +78,7 @@ BOOST_AUTO_TEST_CASE(sign)
         evalScripts[i].SetDestination(standardScripts[i].GetID());
     }
 
-    CTransaction txFrom;  // Funding transaction:
+    CMutableTransaction txFrom;  // Funding transaction:
     string reason;
     txFrom.vout.resize(8);
     for (int i = 0; i < 4; i++)
@@ -90,7 +90,7 @@ BOOST_AUTO_TEST_CASE(sign)
     }
     BOOST_CHECK(IsStandardTx(txFrom, reason));
 
-    CTransaction txTo[8]; // Spending transactions
+    CMutableTransaction txTo[8]; // Spending transactions
     for (int i = 0; i < 8; i++)
     {
         txTo[i].vin.resize(1);
@@ -173,7 +173,7 @@ BOOST_AUTO_TEST_CASE(set)
         keystore.AddCScript(inner[i]);
     }
 
-    CTransaction txFrom;  // Funding transaction:
+    CMutableTransaction txFrom;  // Funding transaction:
     string reason;
     txFrom.vout.resize(4);
     for (int i = 0; i < 4; i++)
@@ -183,7 +183,7 @@ BOOST_AUTO_TEST_CASE(set)
     }
     BOOST_CHECK(IsStandardTx(txFrom, reason));
 
-    CTransaction txTo[4]; // Spending transactions
+    CMutableTransaction txTo[4]; // Spending transactions
     for (int i = 0; i < 4; i++)
     {
         txTo[i].vin.resize(1);
@@ -265,7 +265,7 @@ BOOST_AUTO_TEST_CASE(AreInputsStandard)
         keys.push_back(key[i].GetPubKey());
     }
 
-    CTransaction txFrom;
+    CMutableTransaction txFrom;
     txFrom.vout.resize(6);
 
     // First three are standard:
@@ -299,7 +299,7 @@ BOOST_AUTO_TEST_CASE(AreInputsStandard)
 
     coins.SetCoins(txFrom.GetHash(), CCoins(txFrom, 0));
 
-    CTransaction txTo;
+    CMutableTransaction txTo;
     txTo.vout.resize(1);
     txTo.vout[0].scriptPubKey.SetDestination(key[1].GetPubKey().GetID());
 
@@ -326,7 +326,7 @@ BOOST_AUTO_TEST_CASE(AreInputsStandard)
         txTo.vin[i].scriptSig = t;
     }
 
-    CTransaction txToNonStd;
+    CMutableTransaction txToNonStd;
     txToNonStd.vout.resize(1);
     txToNonStd.vout[0].scriptPubKey.SetDestination(key[1].GetPubKey().GetID());
     txToNonStd.vout[0].nValue = 1000;
diff --git a/src/test/script_tests.cpp b/src/test/script_tests.cpp
index 29d518d70..cba582e94 100644
--- a/src/test/script_tests.cpp
+++ b/src/test/script_tests.cpp
@@ -240,11 +240,11 @@ BOOST_AUTO_TEST_CASE(script_CHECKMULTISIG12)
     CScript scriptPubKey12;
     scriptPubKey12 << OP_1 << key1.GetPubKey() << key2.GetPubKey() << OP_2 << OP_CHECKMULTISIG;
 
-    CTransaction txFrom12;
+    CMutableTransaction txFrom12;
     txFrom12.vout.resize(1);
     txFrom12.vout[0].scriptPubKey = scriptPubKey12;
 
-    CTransaction txTo12;
+    CMutableTransaction txTo12;
     txTo12.vin.resize(1);
     txTo12.vout.resize(1);
     txTo12.vin[0].prevout.n = 0;
@@ -274,11 +274,11 @@ BOOST_AUTO_TEST_CASE(script_CHECKMULTISIG23)
     CScript scriptPubKey23;
     scriptPubKey23 << OP_2 << key1.GetPubKey() << key2.GetPubKey() << key3.GetPubKey() << OP_3 << OP_CHECKMULTISIG;
 
-    CTransaction txFrom23;
+    CMutableTransaction txFrom23;
     txFrom23.vout.resize(1);
     txFrom23.vout[0].scriptPubKey = scriptPubKey23;
 
-    CTransaction txTo23;
+    CMutableTransaction txTo23;
     txTo23.vin.resize(1);
     txTo23.vout.resize(1);
     txTo23.vin[0].prevout.n = 0;
@@ -345,11 +345,11 @@ BOOST_AUTO_TEST_CASE(script_combineSigs)
         keystore.AddKey(key);
     }
 
-    CTransaction txFrom;
+    CMutableTransaction txFrom;
     txFrom.vout.resize(1);
     txFrom.vout[0].scriptPubKey.SetDestination(keys[0].GetPubKey().GetID());
     CScript& scriptPubKey = txFrom.vout[0].scriptPubKey;
-    CTransaction txTo;
+    CMutableTransaction txTo;
     txTo.vin.resize(1);
     txTo.vout.resize(1);
     txTo.vin[0].prevout.n = 0;
diff --git a/src/test/sighash_tests.cpp b/src/test/sighash_tests.cpp
index 353fd60f7..423ae4a78 100644
--- a/src/test/sighash_tests.cpp
+++ b/src/test/sighash_tests.cpp
@@ -28,7 +28,7 @@ uint256 static SignatureHashOld(CScript scriptCode, const CTransaction& txTo, un
         printf("ERROR: SignatureHash() : nIn=%d out of range\n", nIn);
         return 1;
     }
-    CTransaction txTmp(txTo);
+    CMutableTransaction txTmp(txTo);
 
     // In case concatenating two scripts ends up with two codeseparators,
     // or an extra one at the end, this prevents all those possible incompatibilities.
@@ -90,7 +90,7 @@ void static RandomScript(CScript &script) {
         script << oplist[insecure_rand() % (sizeof(oplist)/sizeof(oplist[0]))];
 }
 
-void static RandomTransaction(CTransaction &tx, bool fSingle) {
+void static RandomTransaction(CMutableTransaction &tx, bool fSingle) {
     tx.nVersion = insecure_rand();
     tx.vin.clear();
     tx.vout.clear();
@@ -130,7 +130,7 @@ BOOST_AUTO_TEST_CASE(sighash_test)
     #endif
     for (int i=0; i<nRandomTests; i++) {
         int nHashType = insecure_rand();
-        CTransaction txTo;
+        CMutableTransaction txTo;
         RandomTransaction(txTo, (nHashType & 0x1f) == SIGHASH_SINGLE);
         CScript scriptCode;
         RandomScript(scriptCode);
diff --git a/src/test/transaction_tests.cpp b/src/test/transaction_tests.cpp
index feba5ee80..238033f40 100644
--- a/src/test/transaction_tests.cpp
+++ b/src/test/transaction_tests.cpp
@@ -208,7 +208,7 @@ BOOST_AUTO_TEST_CASE(basic_transaction_tests)
     unsigned char ch[] = {0x01, 0x00, 0x00, 0x00, 0x01, 0x6b, 0xff, 0x7f, 0xcd, 0x4f, 0x85, 0x65, 0xef, 0x40, 0x6d, 0xd5, 0xd6, 0x3d, 0x4f, 0xf9, 0x4f, 0x31, 0x8f, 0xe8, 0x20, 0x27, 0xfd, 0x4d, 0xc4, 0x51, 0xb0, 0x44, 0x74, 0x01, 0x9f, 0x74, 0xb4, 0x00, 0x00, 0x00, 0x00, 0x8c, 0x49, 0x30, 0x46, 0x02, 0x21, 0x00, 0xda, 0x0d, 0xc6, 0xae, 0xce, 0xfe, 0x1e, 0x06, 0xef, 0xdf, 0x05, 0x77, 0x37, 0x57, 0xde, 0xb1, 0x68, 0x82, 0x09, 0x30, 0xe3, 0xb0, 0xd0, 0x3f, 0x46, 0xf5, 0xfc, 0xf1, 0x50, 0xbf, 0x99, 0x0c, 0x02, 0x21, 0x00, 0xd2, 0x5b, 0x5c, 0x87, 0x04, 0x00, 0x76, 0xe4, 0xf2, 0x53, 0xf8, 0x26, 0x2e, 0x76, 0x3e, 0x2d, 0xd5, 0x1e, 0x7f, 0xf0, 0xbe, 0x15, 0x77, 0x27, 0xc4, 0xbc, 0x42, 0x80, 0x7f, 0x17, 0xbd, 0x39, 0x01, 0x41, 0x04, 0xe6, 0xc2, 0x6e, 0xf6, 0x7d, 0xc6, 0x10, 0xd2, 0xcd, 0x19, 0x24, 0x84, 0x78, 0x9a, 0x6c, 0xf9, 0xae, 0xa9, 0x93, 0x0b, 0x94, 0x4b, 0x7e, 0x2d, 0xb5, 0x34, 0x2b, 0x9d, 0x9e, 0x5b, 0x9f, 0xf7, 0x9a, 0xff, 0x9a, 0x2e, 0xe1, 0x97, 0x8d, 0xd7, 0xfd, 0x01, 0xdf, 0xc5, 0x22, 0xee, 0x02, 0x28, 0x3d, 0x3b, 0x06, 0xa9, 0xd0, 0x3a, 0xcf, 0x80, 0x96, 0x96, 0x8d, 0x7d, 0xbb, 0x0f, 0x91, 0x78, 0xff, 0xff, 0xff, 0xff, 0x02, 0x8b, 0xa7, 0x94, 0x0e, 0x00, 0x00, 0x00, 0x00, 0x19, 0x76, 0xa9, 0x14, 0xba, 0xde, 0xec, 0xfd, 0xef, 0x05, 0x07, 0x24, 0x7f, 0xc8, 0xf7, 0x42, 0x41, 0xd7, 0x3b, 0xc0, 0x39, 0x97, 0x2d, 0x7b, 0x88, 0xac, 0x40, 0x94, 0xa8, 0x02, 0x00, 0x00, 0x00, 0x00, 0x19, 0x76, 0xa9, 0x14, 0xc1, 0x09, 0x32, 0x48, 0x3f, 0xec, 0x93, 0xed, 0x51, 0xf5, 0xfe, 0x95, 0xe7, 0x25, 0x59, 0xf2, 0xcc, 0x70, 0x43, 0xf9, 0x88, 0xac, 0x00, 0x00, 0x00, 0x00, 0x00};
     vector<unsigned char> vch(ch, ch + sizeof(ch) -1);
     CDataStream stream(vch, SER_DISK, CLIENT_VERSION);
-    CTransaction tx;
+    CMutableTransaction tx;
     stream >> tx;
     CValidationState state;
     BOOST_CHECK_MESSAGE(CheckTransaction(tx, state) && state.IsValid(), "Simple deserialized transaction should be valid.");
@@ -224,10 +224,10 @@ BOOST_AUTO_TEST_CASE(basic_transaction_tests)
 // paid to a TX_PUBKEY, the second 21 and 22 CENT outputs
 // paid to a TX_PUBKEYHASH.
 //
-static std::vector<CTransaction>
+static std::vector<CMutableTransaction>
 SetupDummyInputs(CBasicKeyStore& keystoreRet, CCoinsView & coinsRet)
 {
-    std::vector<CTransaction> dummyTransactions;
+    std::vector<CMutableTransaction> dummyTransactions;
     dummyTransactions.resize(2);
 
     // Add some keys to the keystore:
@@ -261,9 +261,9 @@ BOOST_AUTO_TEST_CASE(test_Get)
     CBasicKeyStore keystore;
     CCoinsView coinsDummy;
     CCoinsViewCache coins(coinsDummy);
-    std::vector<CTransaction> dummyTransactions = SetupDummyInputs(keystore, coins);
+    std::vector<CMutableTransaction> dummyTransactions = SetupDummyInputs(keystore, coins);
 
-    CTransaction t1;
+    CMutableTransaction t1;
     t1.vin.resize(3);
     t1.vin[0].prevout.hash = dummyTransactions[0].GetHash();
     t1.vin[0].prevout.n = 1;
@@ -296,9 +296,9 @@ BOOST_AUTO_TEST_CASE(test_IsStandard)
     CBasicKeyStore keystore;
     CCoinsView coinsDummy;
     CCoinsViewCache coins(coinsDummy);
-    std::vector<CTransaction> dummyTransactions = SetupDummyInputs(keystore, coins);
+    std::vector<CMutableTransaction> dummyTransactions = SetupDummyInputs(keystore, coins);
 
-    CTransaction t;
+    CMutableTransaction t;
     t.vin.resize(1);
     t.vin[0].prevout.hash = dummyTransactions[0].GetHash();
     t.vin[0].prevout.n = 1;
diff --git a/src/test/wallet_tests.cpp b/src/test/wallet_tests.cpp
index 199335882..86a83f516 100644
--- a/src/test/wallet_tests.cpp
+++ b/src/test/wallet_tests.cpp
@@ -31,16 +31,18 @@ static vector<COutput> vCoins;
 static void add_coin(int64_t nValue, int nAge = 6*24, bool fIsFromMe = false, int nInput=0)
 {
     static int nextLockTime = 0;
-    CTransaction tx;
+    CMutableTransaction tx;
     tx.nLockTime = nextLockTime++;        // so all transactions get different hashes
     tx.vout.resize(nInput+1);
     tx.vout[nInput].nValue = nValue;
+    if (fIsFromMe) {
+        // IsFromMe() returns (GetDebit() > 0), and GetDebit() is 0 if vin.empty(),
+        // so stop vin being empty, and cache a non-zero Debit to fake out IsFromMe()
+        tx.vin.resize(1);
+    }
     CWalletTx* wtx = new CWalletTx(&wallet, tx);
     if (fIsFromMe)
     {
-        // IsFromMe() returns (GetDebit() > 0), and GetDebit() is 0 if vin.empty(),
-        // so stop vin being empty, and cache a non-zero Debit to fake out IsFromMe()
-        wtx->vin.resize(1);
         wtx->fDebitCached = true;
         wtx->nDebitCached = 1;
     }
diff --git a/src/wallet.cpp b/src/wallet.cpp
index 3176cf893..7664d6c25 100644
--- a/src/wallet.cpp
+++ b/src/wallet.cpp
@@ -603,11 +603,11 @@ bool CWallet::AddToWallet(const CWalletTx& wtxIn, bool fFromLoadWallet)
 // Add a transaction to the wallet, or update it.
 // pblock is optional, but should be provided if the transaction is known to be in a block.
 // If fUpdate is true, existing transactions will be updated.
-bool CWallet::AddToWalletIfInvolvingMe(const uint256 &hash, const CTransaction& tx, const CBlock* pblock, bool fUpdate)
+bool CWallet::AddToWalletIfInvolvingMe(const CTransaction& tx, const CBlock* pblock, bool fUpdate)
 {
     {
         AssertLockHeld(cs_wallet);
-        bool fExisted = mapWallet.count(hash);
+        bool fExisted = mapWallet.count(tx.GetHash());
         if (fExisted && !fUpdate) return false;
         if (fExisted || IsMine(tx) || IsFromMe(tx))
         {
@@ -621,10 +621,10 @@ bool CWallet::AddToWalletIfInvolvingMe(const uint256 &hash, const CTransaction&
     return false;
 }
 
-void CWallet::SyncTransaction(const uint256 &hash, const CTransaction& tx, const CBlock* pblock)
+void CWallet::SyncTransaction(const CTransaction& tx, const CBlock* pblock)
 {
     LOCK2(cs_main, cs_wallet);
-    if (!AddToWalletIfInvolvingMe(hash, tx, pblock, true))
+    if (!AddToWalletIfInvolvingMe(tx, pblock, true))
         return; // Not one of ours
 
     // If a transaction changes 'conflicted' state, that changes the balance
@@ -870,7 +870,7 @@ int CWallet::ScanForWalletTransactions(CBlockIndex* pindexStart, bool fUpdate)
             ReadBlockFromDisk(block, pindex);
             BOOST_FOREACH(CTransaction& tx, block.vtx)
             {
-                if (AddToWalletIfInvolvingMe(tx.GetHash(), tx, &block, fUpdate))
+                if (AddToWalletIfInvolvingMe(tx, &block, fUpdate))
                     ret++;
             }
             pindex = chainActive.Next(pindex);
@@ -909,9 +909,8 @@ void CWalletTx::RelayWalletTransaction()
     if (!IsCoinBase())
     {
         if (GetDepthInMainChain() == 0) {
-            uint256 hash = GetHash();
-            LogPrintf("Relaying wtx %s\n", hash.ToString());
-            RelayTransaction((CTransaction)*this, hash);
+            LogPrintf("Relaying wtx %s\n", GetHash().ToString());
+            RelayTransaction((CTransaction)*this);
         }
     }
 }
@@ -1245,6 +1244,7 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
     }
 
     wtxNew.BindWallet(this);
+    CMutableTransaction txNew;
 
     {
         LOCK2(cs_main, cs_wallet);
@@ -1252,8 +1252,8 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
             nFeeRet = payTxFee.GetFeePerK();
             while (true)
             {
-                wtxNew.vin.clear();
-                wtxNew.vout.clear();
+                txNew.vin.clear();
+                txNew.vout.clear();
                 wtxNew.fFromMe = true;
 
                 int64_t nTotalValue = nValue + nFeeRet;
@@ -1267,7 +1267,7 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
                         strFailReason = _("Transaction amount too small");
                         return false;
                     }
-                    wtxNew.vout.push_back(txout);
+                    txNew.vout.push_back(txout);
                 }
 
                 // Choose coins to use
@@ -1331,8 +1331,8 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
                     else
                     {
                         // Insert change txn at random position:
-                        vector<CTxOut>::iterator position = wtxNew.vout.begin()+GetRandInt(wtxNew.vout.size()+1);
-                        wtxNew.vout.insert(position, newTxOut);
+                        vector<CTxOut>::iterator position = txNew.vout.begin()+GetRandInt(txNew.vout.size()+1);
+                        txNew.vout.insert(position, newTxOut);
                     }
                 }
                 else
@@ -1340,17 +1340,20 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
 
                 // Fill vin
                 BOOST_FOREACH(const PAIRTYPE(const CWalletTx*,unsigned int)& coin, setCoins)
-                    wtxNew.vin.push_back(CTxIn(coin.first->GetHash(),coin.second));
+                    txNew.vin.push_back(CTxIn(coin.first->GetHash(),coin.second));
 
                 // Sign
                 int nIn = 0;
                 BOOST_FOREACH(const PAIRTYPE(const CWalletTx*,unsigned int)& coin, setCoins)
-                    if (!SignSignature(*this, *coin.first, wtxNew, nIn++))
+                    if (!SignSignature(*this, *coin.first, txNew, nIn++))
                     {
                         strFailReason = _("Signing transaction failed");
                         return false;
                     }
 
+                // Embed the constructed transaction data in wtxNew.
+                *static_cast<CTransaction*>(&wtxNew) = CTransaction(txNew);
+
                 // Limit size
                 unsigned int nBytes = ::GetSerializeSize(*(CTransaction*)&wtxNew, SER_NETWORK, PROTOCOL_VERSION);
                 if (nBytes >= MAX_STANDARD_TX_SIZE)
diff --git a/src/wallet.h b/src/wallet.h
index 7df656fc2..424799b14 100644
--- a/src/wallet.h
+++ b/src/wallet.h
@@ -244,8 +244,8 @@ public:
 
     void MarkDirty();
     bool AddToWallet(const CWalletTx& wtxIn, bool fFromLoadWallet=false);
-    void SyncTransaction(const uint256 &hash, const CTransaction& tx, const CBlock* pblock);
-    bool AddToWalletIfInvolvingMe(const uint256 &hash, const CTransaction& tx, const CBlock* pblock, bool fUpdate);
+    void SyncTransaction(const CTransaction& tx, const CBlock* pblock);
+    bool AddToWalletIfInvolvingMe(const CTransaction& tx, const CBlock* pblock, bool fUpdate);
     void EraseFromWallet(const uint256 &hash);
     int ScanForWalletTransactions(CBlockIndex* pindexStart, bool fUpdate = false);
     void ReacceptWalletTransactions();