diff --git a/transaction.py b/transaction.py
index 6dbf557..25aa5b2 100644
--- a/transaction.py
+++ b/transaction.py
@@ -1,6 +1,8 @@
 #!/usr/bin/env python3
 import struct
 
+from zc_utils import write_compact_size
+
 MAX_MONEY = 21000000 * 100000000
 TX_EXPIRY_HEIGHT_THRESHOLD = 500000000
 
@@ -89,7 +91,7 @@ class Script(object):
         return self._script
 
     def __bytes__(self):
-        return struct.pack('b', len(self._script)) + self._script
+        return write_compact_size(len(self._script)) + self._script
 
 
 class OutPoint(object):
@@ -167,11 +169,11 @@ class Transaction(object):
             self.nVersionGroupId == OVERWINTER_VERSION_GROUP_ID and \
             self.nVersion == OVERWINTER_TX_VERSION
 
-        ret += struct.pack('b', len(self.vin))
+        ret += write_compact_size(len(self.vin))
         for x in self.vin:
             ret += bytes(x)
 
-        ret += struct.pack('b', len(self.vout))
+        ret += write_compact_size(len(self.vout))
         for x in self.vout:
             ret += bytes(x)
 
@@ -180,7 +182,7 @@ class Transaction(object):
             ret += struct.pack('<I', self.nExpiryHeight)
 
         if self.nVersion >= 2:
-            ret += struct.pack('b', len(self.vJoinSplit))
+            ret += write_compact_size(len(self.vJoinSplit))
             for jsdesc in self.vJoinSplit:
                 ret += bytes(jsdesc)
             if len(self.vJoinSplit) > 0:
diff --git a/zc_utils.py b/zc_utils.py
new file mode 100644
index 0000000..06809ac
--- /dev/null
+++ b/zc_utils.py
@@ -0,0 +1,27 @@
+import struct
+
+def write_compact_size(n):
+    if n < 253:
+        return struct.pack('B', n)
+    elif n <= 0xFFFF:
+        return struct.pack('B', 253) + struct.pack('<H', n)
+    elif n <= 0xFFFFFFFF:
+        return struct.pack('B', 254) + struct.pack('<I', n)
+    else:
+        return struct.pack('B', 255) + struct.pack('<Q', n)
+
+assert write_compact_size(0) == b'\x00'
+assert write_compact_size(1) == b'\x01'
+assert write_compact_size(252) == b'\xFC'
+assert write_compact_size(253) == b'\xFD\xFD\x00'
+assert write_compact_size(254) == b'\xFD\xFE\x00'
+assert write_compact_size(255) == b'\xFD\xFF\x00'
+assert write_compact_size(256) == b'\xFD\x00\x01'
+assert write_compact_size(0xFFFE) == b'\xFD\xFE\xFF'
+assert write_compact_size(0xFFFF) == b'\xFD\xFF\xFF'
+assert write_compact_size(0x010000) == b'\xFE\x00\x00\x01\x00'
+assert write_compact_size(0x010001) == b'\xFE\x01\x00\x01\x00'
+assert write_compact_size(0xFFFFFFFE) == b'\xFE\xFE\xFF\xFF\xFF'
+assert write_compact_size(0xFFFFFFFF) == b'\xFE\xFF\xFF\xFF\xFF'
+assert write_compact_size(0x0100000000) == b'\xFF\x00\x00\x00\x00\x01\x00\x00\x00'
+assert write_compact_size(0xFFFFFFFFFFFFFFFF) == b'\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF'