diff --git a/doc/zmq_socket.txt b/doc/zmq_socket.txt
index 29cb29a8..4f8309c2 100644
--- a/doc/zmq_socket.txt
+++ b/doc/zmq_socket.txt
@@ -160,6 +160,7 @@ and each message sent by Radio sockets belong to a group.
 
 Groups are null terminated strings limited to 16 chars length (including null).
 The intention is to increase the length to 40 chars (including null).
+The encoding of groups shall be UTF8.
 
 Groups are matched using exact matching (vs prefix matching of PubSub).