diff --git a/synapse/handlers/saml_handler.py b/synapse/handlers/saml_handler.py
index ee175a0359..54ba5ebfda 100644
--- a/synapse/handlers/saml_handler.py
+++ b/synapse/handlers/saml_handler.py
@@ -267,6 +267,12 @@ class SamlHandler(BaseHandler):
                     map_username_to_mxid_localpart(attrval), self.server_name
                 ).to_string()
 
+                logger.debug(
+                    "Looking for existing account based on mapped %s %s",
+                    self._grandfathered_mxid_source_attribute,
+                    user_id,
+                )
+
                 users = await self.store.get_users_by_id_case_insensitive(user_id)
                 if users:
                     registered_user_id = list(users.keys())[0]
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
index 65472ab32f..cf7cb7754a 100644
--- a/synapse/handlers/sso.py
+++ b/synapse/handlers/sso.py
@@ -71,6 +71,12 @@ class SsoHandler(BaseHandler):
         Returns:
             The mxid of a previously seen user.
         """
+        logger.debug(
+            "Looking for existing mapping for user %s:%s",
+            auth_provider_id,
+            remote_user_id,
+        )
+
         # Check if we already have a mapping for this user.
         previously_registered_user_id = await self.store.get_user_by_external_id(
             auth_provider_id, remote_user_id,