diff --git a/README b/README
index 103a4574e8..8b91a964b3 100644
--- a/README
+++ b/README
@@ -104,6 +104,7 @@ Your PHP installation must include the following PHP extensions:
 - XMLWriter. This is for formatting XML and HTML output.
 - MySQL. For accessing the database.
 - GD. For scaling down avatar images.
+- mbstring. For handling Unicode (UTF-8) encoded strings.
 
 For some functionality, you will also need the following extensions: