diff --git a/clientapi/routing/sendevent.go b/clientapi/routing/sendevent.go
index e916e451e..9696b360e 100644
--- a/clientapi/routing/sendevent.go
+++ b/clientapi/routing/sendevent.go
@@ -50,7 +50,7 @@ func SendEvent(
 ) util.JSONResponse {
 	if txnID != nil {
 		// Try to fetch response from transactionsCache
-		if res, ok := txnCache.FetchTransaction(*txnID); ok {
+		if res, ok := txnCache.FetchTransaction(device.AccessToken, *txnID); ok {
 			return *res
 		}
 	}
@@ -83,7 +83,7 @@ func SendEvent(
 	}
 	// Add response to transactionsCache
 	if txnID != nil {
-		txnCache.AddTransaction(*txnID, &res)
+		txnCache.AddTransaction(device.AccessToken, *txnID, &res)
 	}
 
 	return res
diff --git a/common/transactions/transactions.go b/common/transactions/transactions.go
index febcb9a75..80b403a98 100644
--- a/common/transactions/transactions.go
+++ b/common/transactions/transactions.go
@@ -22,7 +22,14 @@ import (
 // DefaultCleanupPeriod represents the default time duration after which cacheCleanService runs.
 const DefaultCleanupPeriod time.Duration = 30 * time.Minute
 
-type txnsMap map[string]*util.JSONResponse
+type txnsMap map[CacheKey]*util.JSONResponse
+
+// CacheKey is the type for the key in a transactions cache.
+// This is needed because the spec requires transaction IDs to have a per-access token scope.
+type CacheKey struct {
+	AccessToken string
+	TxnID       string
+}
 
 // Cache represents a temporary store for response entries.
 // Entries are evicted after a certain period, defined by cleanupPeriod.
@@ -50,14 +57,14 @@ func NewWithCleanupPeriod(cleanupPeriod time.Duration) *Cache {
 	return &t
 }
 
-// FetchTransaction looks up an entry for txnID in Cache.
+// FetchTransaction looks up an entry for the (accessToken, txnID) tuple in Cache.
 // Looks in both the txnMaps.
 // Returns (JSON response, true) if txnID is found, else the returned bool is false.
-func (t *Cache) FetchTransaction(txnID string) (*util.JSONResponse, bool) {
+func (t *Cache) FetchTransaction(accessToken, txnID string) (*util.JSONResponse, bool) {
 	t.RLock()
 	defer t.RUnlock()
 	for _, txns := range t.txnsMaps {
-		res, ok := txns[txnID]
+		res, ok := txns[CacheKey{accessToken, txnID}]
 		if ok {
 			return res, true
 		}
@@ -65,13 +72,13 @@ func (t *Cache) FetchTransaction(txnID string) (*util.JSONResponse, bool) {
 	return nil, false
 }
 
-// AddTransaction adds an entry for txnID in Cache for later access.
+// AddTransaction adds an entry for the (accessToken, txnID) tuple in Cache.
 // Adds to the front txnMap.
-func (t *Cache) AddTransaction(txnID string, res *util.JSONResponse) {
+func (t *Cache) AddTransaction(accessToken, txnID string, res *util.JSONResponse) {
 	t.Lock()
 	defer t.Unlock()
 
-	t.txnsMaps[0][txnID] = res
+	t.txnsMaps[0][CacheKey{accessToken, txnID}] = res
 }
 
 // cacheCleanService is responsible for cleaning up entries after cleanupPeriod.
diff --git a/common/transactions/transactions_test.go b/common/transactions/transactions_test.go
index 0cdb776cc..f565e4846 100644
--- a/common/transactions/transactions_test.go
+++ b/common/transactions/transactions_test.go
@@ -24,27 +24,54 @@ type fakeType struct {
 }
 
 var (
-	fakeTxnID    = "aRandomTxnID"
-	fakeResponse = &util.JSONResponse{Code: http.StatusOK, JSON: fakeType{ID: "0"}}
+	fakeAccessToken  = "aRandomAccessToken"
+	fakeAccessToken2 = "anotherRandomAccessToken"
+	fakeTxnID        = "aRandomTxnID"
+	fakeResponse     = &util.JSONResponse{
+		Code: http.StatusOK, JSON: fakeType{ID: "0"},
+	}
+	fakeResponse2 = &util.JSONResponse{
+		Code: http.StatusOK, JSON: fakeType{ID: "1"},
+	}
 )
 
 // TestCache creates a New Cache and tests AddTransaction & FetchTransaction
 func TestCache(t *testing.T) {
 	fakeTxnCache := New()
-	fakeTxnCache.AddTransaction(fakeTxnID, fakeResponse)
+	fakeTxnCache.AddTransaction(fakeAccessToken, fakeTxnID, fakeResponse)
 
 	// Add entries for noise.
 	for i := 1; i <= 100; i++ {
 		fakeTxnCache.AddTransaction(
+			fakeAccessToken,
 			fakeTxnID+string(i),
 			&util.JSONResponse{Code: http.StatusOK, JSON: fakeType{ID: string(i)}},
 		)
 	}
 
-	testResponse, ok := fakeTxnCache.FetchTransaction(fakeTxnID)
+	testResponse, ok := fakeTxnCache.FetchTransaction(fakeAccessToken, fakeTxnID)
 	if !ok {
 		t.Error("Failed to retrieve entry for txnID: ", fakeTxnID)
 	} else if testResponse.JSON != fakeResponse.JSON {
 		t.Error("Fetched response incorrect. Expected: ", fakeResponse.JSON, " got: ", testResponse.JSON)
 	}
 }
+
+// TestCacheScope ensures transactions with the same transaction ID are not shared
+// across multiple access tokens.
+func TestCacheScope(t *testing.T) {
+	cache := New()
+	cache.AddTransaction(fakeAccessToken, fakeTxnID, fakeResponse)
+	cache.AddTransaction(fakeAccessToken2, fakeTxnID, fakeResponse2)
+
+	if res, ok := cache.FetchTransaction(fakeAccessToken, fakeTxnID); !ok {
+		t.Errorf("failed to retrieve entry for (%s, %s)", fakeAccessToken, fakeTxnID)
+	} else if res.JSON != fakeResponse.JSON {
+		t.Errorf("Wrong cache entry for (%s, %s). Expected: %v; got: %v", fakeAccessToken, fakeTxnID, fakeResponse.JSON, res.JSON)
+	}
+	if res, ok := cache.FetchTransaction(fakeAccessToken2, fakeTxnID); !ok {
+		t.Errorf("failed to retrieve entry for (%s, %s)", fakeAccessToken, fakeTxnID)
+	} else if res.JSON != fakeResponse2.JSON {
+		t.Errorf("Wrong cache entry for (%s, %s). Expected: %v; got: %v", fakeAccessToken, fakeTxnID, fakeResponse2.JSON, res.JSON)
+	}
+}
diff --git a/testfile b/testfile
index 81e47780f..5791938f7 100644
--- a/testfile
+++ b/testfile
@@ -159,6 +159,7 @@ Inbound federation rejects remote attempts to kick local users to rooms
 An event which redacts itself should be ignored
 A pair of events which redact each other should be ignored
 Full state sync includes joined rooms
+A message sent after an initial sync appears in the timeline of an incremental sync.
 Can add tag
 Can remove tag
 Can list tags for a room