diff --git a/docs/library/ubinascii.rst b/docs/library/ubinascii.rst
index 0664d5b09db1b80aabef5066d250d14c1dc1ce7c..192d34514b3d951fa933e20b7d2c665e97ea8cdf 100644
--- a/docs/library/ubinascii.rst
+++ b/docs/library/ubinascii.rst
@@ -29,8 +29,12 @@ Functions
 
 .. function:: a2b_base64(data)
 
-   Convert Base64-encoded data to binary representation. Returns bytes string.
+   Decode base64-encoded data, ignoring invalid characters in the input.
+   Conforms to `RFC 2045 s.6.8 <https://tools.ietf.org/html/rfc2045#section-6.8>`_.
+   Returns a bytes object.
 
 .. function:: b2a_base64(data)
 
-   Encode binary data in Base64 format. Returns string.
+   Encode binary data in base64 format, as in `RFC 3548
+   <https://tools.ietf.org/html/rfc3548.html>`_. Returns the encoded data
+   followed by a newline character, as a bytes object.