diff --git a/authlib/oauth2/client.py b/authlib/oauth2/client.py
index d36d93f0..62ea3e49 100644
--- a/authlib/oauth2/client.py
+++ b/authlib/oauth2/client.py
@@ -299,6 +299,8 @@ def revoke_token(self, url, token=None, token_type_hint=None,
 
         .. _`RFC7009`: https://tools.ietf.org/html/rfc7009
         """
+        if auth is None:
+            auth = self.client_auth(self.revocation_endpoint_auth_method)
         return self._handle_token_hint(
             'revoke_token_request', url,
             token=token, token_type_hint=token_type_hint,
@@ -320,6 +322,8 @@ def introspect_token(self, url, token=None, token_type_hint=None,
 
         .. _`RFC7662`: https://tools.ietf.org/html/rfc7662
         """
+        if auth is None:
+            auth = self.client_auth(self.token_endpoint_auth_method)
         return self._handle_token_hint(
             'introspect_token_request', url,
             token=token, token_type_hint=token_type_hint,