diff --git a/src/Lifecycle.js b/src/Lifecycle.js
index 493bbf12aa..64d22bd04c 100644
--- a/src/Lifecycle.js
+++ b/src/Lifecycle.js
@@ -22,6 +22,7 @@ import Notifier from './Notifier';
 import UserActivity from './UserActivity';
 import Presence from './Presence';
 import dis from './dispatcher';
+import Modal from './Modal';
 import DMRoomMap from './utils/DMRoomMap';
 
 /**
@@ -289,19 +290,35 @@ export function logout() {
         return;
     }
 
-    return MatrixClientPeg.get().logout().then(onLoggedOut,
-        (err) => {
-            // Just throwing an error here is going to be very unhelpful
-            // if you're trying to log out because your server's down and
-            // you want to log into a different server, so just forget the
-            // access token. It's annoying that this will leave the access
-            // token still valid, but we should fix this by having access
-            // tokens expire (and if you really think you've been compromised,
-            // change your password).
-            console.log("Failed to call logout API: token will not be invalidated");
-            onLoggedOut();
-        }
-    );
+    var QuestionDialog = sdk.getComponent("dialogs.QuestionDialog");
+    Modal.createDialog(QuestionDialog, {
+        title: "Warning",
+        description:
+            <div>
+                For security, logging out will delete any end-to-end encryption keys from this browser,
+                making previous encrypted chat history unreadable if you log back in.
+                In future this <a href="https://github.com/vector-im/riot-web/issues/2108">will be improved</a>,
+                but for now be warned.
+            </div>,
+        button: "Continue",
+        onFinished: (confirmed) => {
+            if (confirmed) {
+                MatrixClientPeg.get().logout().then(onLoggedOut,
+                    (err) => {
+                        // Just throwing an error here is going to be very unhelpful
+                        // if you're trying to log out because your server's down and
+                        // you want to log into a different server, so just forget the
+                        // access token. It's annoying that this will leave the access
+                        // token still valid, but we should fix this by having access
+                        // tokens expire (and if you really think you've been compromised,
+                        // change your password).
+                        console.log("Failed to call logout API: token will not be invalidated");
+                        onLoggedOut();
+                    }
+                );
+            }
+        },
+    });
 }
 
 /**