diff --git a/synapse/rest/base.py b/synapse/rest/base.py
index 6a88cbe86..e855d293e 100644
--- a/synapse/rest/base.py
+++ b/synapse/rest/base.py
@@ -15,6 +15,7 @@
 
 """ This module contains base REST classes for constructing REST servlets. """
 from synapse.api.urls import CLIENT_PREFIX
+from synapse.rest.transactions import HttpTransactionStore
 import re
 
 
@@ -59,6 +60,7 @@ class RestServlet(object):
         self.handlers = hs.get_handlers()
         self.event_factory = hs.get_event_factory()
         self.auth = hs.get_auth()
+        self.txns = HttpTransactionStore()
 
     def register(self, http_server):
         """ Register this servlet with the given HTTP server. """
diff --git a/synapse/rest/room.py b/synapse/rest/room.py
index 118208ddc..4ca7e7e78 100644
--- a/synapse/rest/room.py
+++ b/synapse/rest/room.py
@@ -375,10 +375,16 @@ class RoomMembershipRestServlet(RestServlet):
         register_txn_path(self, PATTERN, http_server)
 
     def on_POST(self, request, room_id, membership_action):
-        return (200, "Not implemented")
+        return (200, "POST Not implemented")
 
     def on_PUT(self, request, room_id, membership_action, txn_id):
-        return (200, "Not implemented")
+        (code, response) = self.txns.get_client_transaction(request, txn_id)
+        if code:
+            return (code, response)
+
+        response = (200, "PUT not implemented txnid %s" % txn_id)
+        self.txns.store_client_transaction(request, txn_id, response)
+        return response
 
 
 def _parse_json(request):
diff --git a/synapse/rest/transactions.py b/synapse/rest/transactions.py
new file mode 100644
index 000000000..10be10e90
--- /dev/null
+++ b/synapse/rest/transactions.py
@@ -0,0 +1,93 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014 matrix.org
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""This module contains logic for storing HTTP PUT transactions. This is used
+to ensure idempotency when performing PUTs using the REST API."""
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+class HttpTransactionStore(object):
+
+    def __init__(self):
+        # { key : (txn_id, response) }
+        self.transactions = {}
+
+    def get_response(self, key, txn_id):
+        """Retrieve a response for this request.
+
+        Args:
+            key (str): A transaction-independent key for this request. Typically
+            this is a combination of the path (without the transaction id) and
+            the user's access token.
+            txn_id (str): The transaction ID for this request
+        Returns:
+            A tuple of (HTTP response code, response content) or None.
+        """
+        try:
+            logger.debug("get_response Key: %s TxnId: %s", key, txn_id)
+            (last_txn_id, response) = self.transactions[key]
+            if txn_id == last_txn_id:
+                return response
+        except KeyError:
+            pass
+        return None
+
+    def store_response(self, key, txn_id, response):
+        """Stores an HTTP response tuple.
+
+        Args:
+            key (str): A transaction-independent key for this request. Typically
+            this is a combination of the path (without the transaction id) and
+            the user's access token.
+            txn_id (str): The transaction ID for this request.
+            response (tuple): A tuple of (HTTP response code, response content)
+        """
+        logger.debug("store_response Key: %s TxnId: %s", key, txn_id)
+        self.transactions[key] = (txn_id, response)
+
+    def store_client_transaction(self, request, txn_id, response):
+        """Stores the request/response pair of an HTTP transaction.
+
+        Args:
+            request (twisted.web.http.Request): The twisted HTTP request. This
+            request must have the transaction ID as the last path segment.
+            response (tuple): A tuple of (response code, response dict)
+            txn_id (str): The transaction ID for this request.
+        """
+        self.store_response(self._get_key(request), txn_id, response)
+
+    def get_client_transaction(self, request, txn_id):
+        """Retrieves a stored response if there was one.
+
+        Args:
+            request (twisted.web.http.Request): The twisted HTTP request. This
+            request must have the transaction ID as the last path segment.
+            txn_id (str): The transaction ID for this request.
+        Returns:
+            The response tuple or (None, None).
+        """
+        response = self.get_response(self._get_key(request), txn_id)
+        if response is None:
+            return (None, None)
+        return response
+
+    def _get_key(self, request):
+        token = request.args["access_token"][0]
+        path_without_txn_id = request.path.rsplit("/", 1)[0]
+        return path_without_txn_id + "/" + token
+
+