diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 8c41cb6f3..b6ef3c91a 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -24,6 +24,7 @@ from synapse.api.constants import (
 )
 from synapse.api.errors import AuthError, SynapseError, Codes
 from synapse.util.logcontext import preserve_context_over_fn
+from synapse.util.async import Linearizer
 
 from signedjson.sign import verify_signed_json
 from signedjson.key import decode_verify_key_bytes
@@ -60,6 +61,8 @@ class RoomMemberHandler(BaseHandler):
     def __init__(self, hs):
         super(RoomMemberHandler, self).__init__(hs)
 
+        self.member_linearizer = Linearizer()
+
         self.clock = hs.get_clock()
 
         self.distributor = hs.get_distributor()
@@ -182,6 +185,34 @@ class RoomMemberHandler(BaseHandler):
             remote_room_hosts=None,
             third_party_signed=None,
             ratelimit=True,
+    ):
+        key = (target, room_id,)
+
+        with (yield self.member_linearizer.queue(key)):
+            result = yield self._update_membership(
+                requester,
+                target,
+                room_id,
+                action,
+                txn_id=txn_id,
+                remote_room_hosts=remote_room_hosts,
+                third_party_signed=third_party_signed,
+                ratelimit=ratelimit,
+            )
+
+        defer.returnValue(result)
+
+    @defer.inlineCallbacks
+    def _update_membership(
+            self,
+            requester,
+            target,
+            room_id,
+            action,
+            txn_id=None,
+            remote_room_hosts=None,
+            third_party_signed=None,
+            ratelimit=True,
     ):
         effective_membership_state = action
         if action in ["kick", "unban"]:
diff --git a/synapse/util/async.py b/synapse/util/async.py
index cd4d90f3c..0d6f48e2d 100644
--- a/synapse/util/async.py
+++ b/synapse/util/async.py
@@ -16,9 +16,13 @@
 
 from twisted.internet import defer, reactor
 
-from .logcontext import PreserveLoggingContext, preserve_fn
+from .logcontext import (
+    PreserveLoggingContext, preserve_fn, preserve_context_over_deferred,
+)
 from synapse.util import unwrapFirstError
 
+from contextlib import contextmanager
+
 
 @defer.inlineCallbacks
 def sleep(seconds):
@@ -137,3 +141,47 @@ def concurrently_execute(func, args, limit):
         preserve_fn(_concurrently_execute_inner)()
         for _ in xrange(limit)
     ], consumeErrors=True).addErrback(unwrapFirstError)
+
+
+class Linearizer(object):
+    """Linearizes access to resources based on a key. Useful to ensure only one
+    thing is happening at a time on a given resource.
+
+    Example:
+
+        with (yield linearizer.queue("test_key")):
+            # do some work.
+
+    """
+    def __init__(self):
+        self.key_to_defer = {}
+
+    @defer.inlineCallbacks
+    def queue(self, key):
+        # If there is already a deferred in the queue, we pull it out so that
+        # we can wait on it later.
+        # Then we replace it with a deferred that we resolve *after* the
+        # context manager has exited.
+        # We only return the context manager after the previous deferred has
+        # resolved.
+        # This all has the net effect of creating a chain of deferreds that
+        # wait for the previous deferred before starting their work.
+        current_defer = self.key_to_defer.get(key)
+
+        new_defer = defer.Deferred()
+        self.key_to_defer[key] = new_defer
+
+        if current_defer:
+            yield preserve_context_over_deferred(current_defer)
+
+        @contextmanager
+        def _ctx_manager():
+            try:
+                yield
+            finally:
+                new_defer.callback(None)
+                current_d = self.key_to_defer.get(key)
+                if current_d is new_defer:
+                    self.key_to_defer.pop(key, None)
+
+        defer.returnValue(_ctx_manager())
diff --git a/synapse/util/caches/response_cache.py b/synapse/util/caches/response_cache.py
index be310ba32..36686b479 100644
--- a/synapse/util/caches/response_cache.py
+++ b/synapse/util/caches/response_cache.py
@@ -35,7 +35,7 @@ class ResponseCache(object):
             return None
 
     def set(self, key, deferred):
-        result = ObservableDeferred(deferred)
+        result = ObservableDeferred(deferred, consumeErrors=True)
         self.pending_result_cache[key] = result
 
         def remove(r):
diff --git a/tests/util/test_linearizer.py b/tests/util/test_linearizer.py
new file mode 100644
index 000000000..afcba482f
--- /dev/null
+++ b/tests/util/test_linearizer.py
@@ -0,0 +1,44 @@
+# -*- coding: utf-8 -*-
+# Copyright 2016 OpenMarket Ltd
+#
+# 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.
+
+
+from tests import unittest
+
+from twisted.internet import defer
+
+from synapse.util.async import Linearizer
+
+
+class LinearizerTestCase(unittest.TestCase):
+
+    @defer.inlineCallbacks
+    def test_linearizer(self):
+        linearizer = Linearizer()
+
+        key = object()
+
+        d1 = linearizer.queue(key)
+        cm1 = yield d1
+
+        d2 = linearizer.queue(key)
+        self.assertFalse(d2.called)
+
+        with cm1:
+            self.assertFalse(d2.called)
+
+        self.assertTrue(d2.called)
+
+        with (yield d2):
+            pass