diff --git a/core/src/main/kotlin/com/r3corda/core/contracts/TransactionGraphSearch.kt b/core/src/main/kotlin/com/r3corda/core/contracts/TransactionGraphSearch.kt
index 98fe1bd369..404dda5a3e 100644
--- a/core/src/main/kotlin/com/r3corda/core/contracts/TransactionGraphSearch.kt
+++ b/core/src/main/kotlin/com/r3corda/core/contracts/TransactionGraphSearch.kt
@@ -45,7 +45,7 @@ class TransactionGraphSearch(val transactions: ReadOnlyTransactionStorage,
             val unvisitedInputTxs: Map<SecureHash, SignedTransaction> = inputsLeadingToUnvisitedTx.map { it.txhash }.toHashSet().map { transactions.getTransaction(it) }.filterNotNull().associateBy { it.id }
             val unvisitedInputTxsWithInputIndex: Iterable<Pair<SignedTransaction, Int>> = inputsLeadingToUnvisitedTx.filter { it.txhash in unvisitedInputTxs.keys }.map { Pair(unvisitedInputTxs[it.txhash]!!, it.index) }
             next += (unvisitedInputTxsWithInputIndex.filter { q.followInputsOfType == null || it.first.tx.outputs[it.second].data.javaClass == q.followInputsOfType }
-                    .map { it.first }.filter { alreadyVisited.add(it.txBits.hash) }.map { it.tx })
+                    .map { it.first }.filter { alreadyVisited.add(it.id) }.map { it.tx })
         }
 
         return results
diff --git a/core/src/main/kotlin/com/r3corda/core/transactions/SignedTransaction.kt b/core/src/main/kotlin/com/r3corda/core/transactions/SignedTransaction.kt
index 10040cd6fd..4ed2ed4fcb 100644
--- a/core/src/main/kotlin/com/r3corda/core/transactions/SignedTransaction.kt
+++ b/core/src/main/kotlin/com/r3corda/core/transactions/SignedTransaction.kt
@@ -19,9 +19,12 @@ import java.util.*
  * of a WireTransaction, therefore if you are storing data keyed by WT hash be aware that multiple different STs may
  * map to the same key (and they could be different in important ways, like validity!). The signatures on a
  * SignedTransaction might be invalid or missing: the type does not imply validity.
+ * A transaction ID should be the hash of the [WireTransaction] Merkle tree root. Thus adding or removing a signature does not change it.
  */
 data class SignedTransaction(val txBits: SerializedBytes<WireTransaction>,
-                             val sigs: List<DigitalSignature.WithKey>) : NamedByHash {
+                             val sigs: List<DigitalSignature.WithKey>,
+                             override val id: SecureHash
+) : NamedByHash {
     init {
         require(sigs.isNotEmpty())
     }
@@ -31,9 +34,6 @@ data class SignedTransaction(val txBits: SerializedBytes<WireTransaction>,
     /** Lazily calculated access to the deserialised/hashed transaction data. */
     val tx: WireTransaction by lazy { WireTransaction.deserialize(txBits) }
 
-    /** A transaction ID is the hash of the [WireTransaction]. Thus adding or removing a signature does not change it. */
-    override val id: SecureHash get() = tx.id
-
     class SignaturesMissingException(val missing: Set<PublicKey>, val descriptions: List<String>, override val id: SecureHash) : NamedByHash, SignatureException() {
         override fun toString(): String {
             return "Missing signatures for $descriptions on transaction ${id.prefixChars()} for ${missing.toStringsShort()}"
@@ -64,6 +64,7 @@ data class SignedTransaction(val txBits: SerializedBytes<WireTransaction>,
             if (needed.isNotEmpty())
                 throw SignaturesMissingException(needed, getMissingKeyDescriptions(needed), id)
         }
+        check(tx.id == id)
         return tx
     }
 
@@ -77,8 +78,9 @@ data class SignedTransaction(val txBits: SerializedBytes<WireTransaction>,
      */
     @Throws(SignatureException::class)
     fun checkSignaturesAreValid() {
-        for (sig in sigs)
-            sig.verifyWithECDSA(txBits.bits)
+        for (sig in sigs) {
+            sig.verifyWithECDSA(id.bits)
+        }
     }
 
     private fun getMissingSignatures(): Set<PublicKey> {
diff --git a/core/src/main/kotlin/com/r3corda/core/transactions/TransactionBuilder.kt b/core/src/main/kotlin/com/r3corda/core/transactions/TransactionBuilder.kt
index 34d5e0f623..c6aefc7689 100644
--- a/core/src/main/kotlin/com/r3corda/core/transactions/TransactionBuilder.kt
+++ b/core/src/main/kotlin/com/r3corda/core/transactions/TransactionBuilder.kt
@@ -99,7 +99,7 @@ open class TransactionBuilder(
 
     fun signWith(key: KeyPair): TransactionBuilder {
         check(currentSigs.none { it.by == key.public }) { "This partial transaction was already signed by ${key.public}" }
-        val data = toWireTransaction().serialize()
+        val data = toWireTransaction().id
         addSignatureUnchecked(key.signWithECDSA(data.bits))
         return this
     }
@@ -124,7 +124,7 @@ open class TransactionBuilder(
      */
     fun checkSignature(sig: DigitalSignature.WithKey) {
         require(commands.any { it.signers.contains(sig.by) }) { "Signature key doesn't match any command" }
-        sig.verifyWithECDSA(toWireTransaction().serialized)
+        sig.verifyWithECDSA(toWireTransaction().id)
     }
 
     /** Adds the signature directly to the transaction, without checking it for validity. */
@@ -143,7 +143,8 @@ open class TransactionBuilder(
             if (missing.isNotEmpty())
                 throw IllegalStateException("Missing signatures on the transaction for the public keys: ${missing.toStringsShort()}")
         }
-        return SignedTransaction(toWireTransaction().serialize(), ArrayList(currentSigs))
+        val wtx = toWireTransaction()
+        return SignedTransaction(wtx.serialize(), ArrayList(currentSigs), wtx.id)
     }
 
     open fun addInputState(stateAndRef: StateAndRef<*>) {
diff --git a/core/src/main/kotlin/com/r3corda/protocols/AbstractStateReplacementProtocol.kt b/core/src/main/kotlin/com/r3corda/protocols/AbstractStateReplacementProtocol.kt
index c702791765..ff3cc2f914 100644
--- a/core/src/main/kotlin/com/r3corda/protocols/AbstractStateReplacementProtocol.kt
+++ b/core/src/main/kotlin/com/r3corda/protocols/AbstractStateReplacementProtocol.kt
@@ -96,7 +96,7 @@ abstract class AbstractStateReplacementProtocol<T> {
                 if (it.sig == null) throw StateReplacementException(it.error!!)
                 else {
                     check(it.sig.by == party.owningKey) { "Not signed by the required participant" }
-                    it.sig.verifyWithECDSA(stx.txBits)
+                    it.sig.verifyWithECDSA(stx.id)
                     it.sig
                 }
             }
@@ -154,7 +154,7 @@ abstract class AbstractStateReplacementProtocol<T> {
 
             // TODO: This step should not be necessary, as signatures are re-checked in verifySignatures.
             val allSignatures = swapSignatures.unwrap { signatures ->
-                signatures.forEach { it.verifyWithECDSA(stx.txBits) }
+                signatures.forEach { it.verifyWithECDSA(stx.id) }
                 signatures
             }
 
@@ -199,7 +199,7 @@ abstract class AbstractStateReplacementProtocol<T> {
 
         private fun sign(stx: SignedTransaction): DigitalSignature.WithKey {
             val myKey = serviceHub.legalIdentityKey
-            return myKey.signWithECDSA(stx.txBits)
+            return myKey.signWithECDSA(stx.id)
         }
     }
 
diff --git a/core/src/main/kotlin/com/r3corda/protocols/NotaryProtocol.kt b/core/src/main/kotlin/com/r3corda/protocols/NotaryProtocol.kt
index 528decd434..109391fa29 100644
--- a/core/src/main/kotlin/com/r3corda/protocols/NotaryProtocol.kt
+++ b/core/src/main/kotlin/com/r3corda/protocols/NotaryProtocol.kt
@@ -66,7 +66,7 @@ object NotaryProtocol {
                 progressTracker.currentStep = VALIDATING
                 when (notaryResult) {
                     is Result.Success -> {
-                        validateSignature(notaryResult.sig, stx.txBits)
+                        validateSignature(notaryResult.sig, stx.id.bits)
                         notaryResult.sig
                     }
                     is Result.Error -> {
@@ -79,7 +79,7 @@ object NotaryProtocol {
             }
         }
 
-        private fun validateSignature(sig: DigitalSignature.LegallyIdentifiable, data: SerializedBytes<WireTransaction>) {
+        private fun validateSignature(sig: DigitalSignature.LegallyIdentifiable, data: ByteArray) {
             check(sig.signer == notaryParty) { "Notary result not signed by the correct service" }
             sig.verifyWithECDSA(data)
         }
@@ -108,7 +108,7 @@ object NotaryProtocol {
                 beforeCommit(stx, reqIdentity)
                 commitInputStates(wtx, reqIdentity)
 
-                val sig = sign(stx.txBits)
+                val sig = sign(stx.id.bits)
                 Result.Success(sig)
             } catch(e: NotaryException) {
                 Result.Error(e.error)
@@ -140,12 +140,12 @@ object NotaryProtocol {
                 uniquenessProvider.commit(tx.inputs, tx.id, reqIdentity)
             } catch (e: UniquenessException) {
                 val conflictData = e.error.serialize()
-                val signedConflict = SignedData(conflictData, sign(conflictData))
+                val signedConflict = SignedData(conflictData, sign(conflictData.bits))
                 throw NotaryException(NotaryError.Conflict(tx, signedConflict))
             }
         }
 
-        private fun <T : Any> sign(bits: SerializedBytes<T>): DigitalSignature.LegallyIdentifiable {
+        private fun sign(bits: ByteArray): DigitalSignature.LegallyIdentifiable {
             val myNodeInfo = serviceHub.myInfo
             val myIdentity = myNodeInfo.notaryIdentity
             val mySigningKey = serviceHub.notaryIdentityKey
diff --git a/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt b/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt
index 8f8d32c388..7dd805fdd7 100644
--- a/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt
+++ b/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt
@@ -170,7 +170,7 @@ object TwoPartyDealProtocol {
 
         open fun computeOurSignature(partialTX: SignedTransaction): DigitalSignature.WithKey {
             progressTracker.currentStep = SIGNING
-            return myKeyPair.signWithECDSA(partialTX.txBits)
+            return myKeyPair.signWithECDSA(partialTX.id)
         }
 
         @Suspendable
diff --git a/core/src/test/kotlin/com/r3corda/core/contracts/TransactionTests.kt b/core/src/test/kotlin/com/r3corda/core/contracts/TransactionTests.kt
index 0e7b718d95..c5e4a3216e 100644
--- a/core/src/test/kotlin/com/r3corda/core/contracts/TransactionTests.kt
+++ b/core/src/test/kotlin/com/r3corda/core/contracts/TransactionTests.kt
@@ -34,7 +34,7 @@ class TransactionTests {
                 timestamp = null
         )
         val bits: SerializedBytes<WireTransaction> = wtx.serialized
-        fun make(vararg keys: KeyPair) = SignedTransaction(bits, keys.map { it.signWithECDSA(bits) })
+        fun make(vararg keys: KeyPair) = SignedTransaction(bits, keys.map { it.signWithECDSA(wtx.id.bits) }, wtx.id)
         assertFailsWith<IllegalArgumentException> { make().verifySignatures() }
 
         assertEquals(
diff --git a/core/src/test/kotlin/com/r3corda/core/serialization/TransactionSerializationTests.kt b/core/src/test/kotlin/com/r3corda/core/serialization/TransactionSerializationTests.kt
index e29fb02903..ea5b18e84b 100644
--- a/core/src/test/kotlin/com/r3corda/core/serialization/TransactionSerializationTests.kt
+++ b/core/src/test/kotlin/com/r3corda/core/serialization/TransactionSerializationTests.kt
@@ -67,7 +67,7 @@ class TransactionSerializationTests {
         signedTX.verifySignatures()
 
         // Corrupt the data and ensure the signature catches the problem.
-        signedTX.txBits.bits[5] = 0
+        signedTX.id.bits[5] = 0
         assertFailsWith(SignatureException::class) {
             signedTX.verifySignatures()
         }
diff --git a/finance/src/main/kotlin/com/r3corda/contracts/testing/Generators.kt b/finance/src/main/kotlin/com/r3corda/contracts/testing/Generators.kt
index 765d614ffd..5175e8884c 100644
--- a/finance/src/main/kotlin/com/r3corda/contracts/testing/Generators.kt
+++ b/finance/src/main/kotlin/com/r3corda/contracts/testing/Generators.kt
@@ -81,7 +81,8 @@ class SignedTransactionGenerator: Generator<SignedTransaction>(SignedTransaction
         val wireTransaction = WiredTransactionGenerator().generate(random, status)
         return SignedTransaction(
                 txBits = wireTransaction.serialized,
-                sigs = listOf(NullSignature)
+                sigs = listOf(NullSignature),
+                id = wireTransaction.id
         )
     }
 }
diff --git a/finance/src/main/kotlin/com/r3corda/protocols/TwoPartyTradeProtocol.kt b/finance/src/main/kotlin/com/r3corda/protocols/TwoPartyTradeProtocol.kt
index 8ce6d421b8..a943d9f811 100644
--- a/finance/src/main/kotlin/com/r3corda/protocols/TwoPartyTradeProtocol.kt
+++ b/finance/src/main/kotlin/com/r3corda/protocols/TwoPartyTradeProtocol.kt
@@ -141,7 +141,7 @@ object TwoPartyTradeProtocol {
 
         open fun calculateOurSignature(partialTX: SignedTransaction): DigitalSignature.WithKey {
             progressTracker.currentStep = SIGNING
-            return myKeyPair.signWithECDSA(partialTX.txBits)
+            return myKeyPair.signWithECDSA(partialTX.id)
         }
 
         @Suspendable
diff --git a/node/src/test/kotlin/com/r3corda/node/services/NotaryServiceTests.kt b/node/src/test/kotlin/com/r3corda/node/services/NotaryServiceTests.kt
index 158fad5005..7443574fe8 100644
--- a/node/src/test/kotlin/com/r3corda/node/services/NotaryServiceTests.kt
+++ b/node/src/test/kotlin/com/r3corda/node/services/NotaryServiceTests.kt
@@ -54,7 +54,7 @@ class NotaryServiceTests {
 
         val future = runNotaryClient(stx)
         val signature = future.get()
-        signature.verifyWithECDSA(stx.txBits)
+        signature.verifyWithECDSA(stx.id)
     }
 
     @Test fun `should sign a unique transaction without a timestamp`() {
@@ -67,7 +67,7 @@ class NotaryServiceTests {
 
         val future = runNotaryClient(stx)
         val signature = future.get()
-        signature.verifyWithECDSA(stx.txBits)
+        signature.verifyWithECDSA(stx.id)
     }
 
     @Test fun `should report error for transaction with an invalid timestamp`() {
diff --git a/node/src/test/kotlin/com/r3corda/node/services/persistence/DBTransactionStorageTests.kt b/node/src/test/kotlin/com/r3corda/node/services/persistence/DBTransactionStorageTests.kt
index a1732fc7a6..ed46e7ca03 100644
--- a/node/src/test/kotlin/com/r3corda/node/services/persistence/DBTransactionStorageTests.kt
+++ b/node/src/test/kotlin/com/r3corda/node/services/persistence/DBTransactionStorageTests.kt
@@ -1,13 +1,11 @@
 package com.r3corda.node.services.persistence
 
-import com.google.common.primitives.Ints
 import com.google.common.util.concurrent.SettableFuture
 import com.r3corda.core.contracts.StateRef
 import com.r3corda.core.contracts.TransactionType
 import com.r3corda.core.crypto.DigitalSignature
 import com.r3corda.core.crypto.NullPublicKey
 import com.r3corda.core.crypto.SecureHash
-import com.r3corda.core.serialization.SerializedBytes
 import com.r3corda.core.transactions.SignedTransaction
 import com.r3corda.core.transactions.WireTransaction
 import com.r3corda.core.utilities.DUMMY_NOTARY
@@ -144,6 +142,6 @@ class DBTransactionStorageTests {
                 type = TransactionType.General(),
                 timestamp = null
         )
-        return SignedTransaction(wtx.serialized, listOf(DigitalSignature.WithKey(NullPublicKey, ByteArray(1))))
+        return SignedTransaction(wtx.serialized, listOf(DigitalSignature.WithKey(NullPublicKey, ByteArray(1))), wtx.id)
     }
 }
\ No newline at end of file
diff --git a/test-utils/src/main/kotlin/com/r3corda/testing/TestDSL.kt b/test-utils/src/main/kotlin/com/r3corda/testing/TestDSL.kt
index d73bb6ad1d..dcf30c25ec 100644
--- a/test-utils/src/main/kotlin/com/r3corda/testing/TestDSL.kt
+++ b/test-utils/src/main/kotlin/com/r3corda/testing/TestDSL.kt
@@ -279,10 +279,10 @@ data class TestLedgerDSLInterpreter private constructor (
 
     override fun verifies(): EnforceVerifyOrFail {
         try {
-            services.recordTransactions(transactionsUnverified.map { SignedTransaction(it.serialized, listOf(NullSignature)) })
+            services.recordTransactions(transactionsUnverified.map { SignedTransaction(it.serialized, listOf(NullSignature), it.id) })
             for ((key, value) in transactionWithLocations) {
                 value.transaction.toLedgerTransaction(services).verify()
-                services.recordTransactions(SignedTransaction(value.transaction.serialized, listOf(NullSignature)))
+                services.recordTransactions(SignedTransaction(value.transaction.serialized, listOf(NullSignature), value.transaction.id))
             }
             return EnforceVerifyOrFail.Token
         } catch (exception: TransactionVerificationException) {
@@ -326,9 +326,9 @@ fun signAll(transactionsToSign: List<WireTransaction>, extraKeys: List<KeyPair>)
     }
     wtx.mustSign.forEach {
         val key = keyLookup[it] ?: throw IllegalArgumentException("Missing required key for ${it.toStringShort()}")
-        signatures += key.signWithECDSA(bits)
+        signatures += key.signWithECDSA(wtx.id)
     }
-    SignedTransaction(bits, signatures)
+    SignedTransaction(bits, signatures, wtx.id)
 }
 
 /**