diff --git a/src/vector/olm-loader.js b/src/vector/olm-loader.js
new file mode 100644
index 0000000000..a62d05b40e
--- /dev/null
+++ b/src/vector/olm-loader.js
@@ -0,0 +1,41 @@
+/*
+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.
+*/
+
+/* a very thin shim for loading olm.js: just sets the global OLM_OPTIONS and
+ * requires the actual olm.js library.
+ *
+ * olm.js reads global.OLM_OPTIONS and defines global.Olm. The latter is fine for us,
+ * but we need to prepare the former.
+ *
+ * We can't use webpack's definePlugin to do this, because we tell webpack not
+ * to parse olm.js. We also can't put this code in index.js, because olm and
+ * index.js are loaded in parallel, and we need to make sure OLM_OPTIONS is set
+ * before olm.js is loaded.
+ */
+
+/* total_memory must be a power of two, and at least twice the stack.
+ *
+ * We don't need a lot of stack, but we do need about 128K of heap to encrypt a
+ * 64K event (enough to store the ciphertext and the plaintext, bearing in mind
+ * that the plaintext can only be 48K because base64). We also have about 36K
+ * of statics. So let's have 256K of memory.
+ */
+global.OLM_OPTIONS = {
+    TOTAL_STACK: 64*1024,
+    TOTAL_MEMORY: 256*1024,
+};
+
+require('olm/olm.js');
diff --git a/webpack.config.js b/webpack.config.js
index cad7b9a38d..3500fedcb6 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -14,7 +14,7 @@ module.exports = {
         // (we should probably make js-sdk load it asynchronously at some
         // point, so that it doesn't block the pageload, but that is a separate
         // problem)
-        "olm": "olm/olm.js",
+        "olm": "./src/vector/olm-loader.js",
     },
     module: {
         preLoaders: [