diff --git a/docs/megolm.rst b/docs/megolm.rst
index 6d20cff..7853963 100644
--- a/docs/megolm.rst
+++ b/docs/megolm.rst
@@ -243,7 +243,8 @@ signature.
 
 The version byte, ``V``, is ``"\x03"``.
 
-The payload consists of the following key-value pairs:
+The payload uses a format based on the `Protocol Buffers encoding`_. It
+consists of the following key-value pairs:
 
 ============= ===== ======== ================================================
     Name       Tag    Type                     Meaning
@@ -286,3 +287,4 @@ Version 2.0 <http://www.apache.org/licenses/LICENSE-2.0>`_.
 .. _`CBC`: http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
 .. _`PCKS#7`: https://tools.ietf.org/html/rfc2315
 .. _`Olm`: ./olm.html
+.. _`Protocol Buffers encoding`: https://developers.google.com/protocol-buffers/docs/encoding