diff --git a/libs/gtkmm2ext/gtk_ui.cc b/libs/gtkmm2ext/gtk_ui.cc
index 734a93c793..7695892a82 100644
--- a/libs/gtkmm2ext/gtk_ui.cc
+++ b/libs/gtkmm2ext/gtk_ui.cc
@@ -369,6 +369,10 @@ UI::idle_add (int (*func)(void *), void *arg)
 
 /* END abstract_ui interfaces */
 
+/** Create a PBD::EventLoop::InvalidationRecord and attach a callback
+ *  to a given sigc::trackable so that PBD::EventLoop::invalidate_request
+ *  is called when that trackable is destroyed.
+ */
 PBD::EventLoop::InvalidationRecord*
 __invalidator (sigc::trackable& trackable, const char* file, int line)
 {
diff --git a/libs/pbd/event_loop.cc b/libs/pbd/event_loop.cc
index 69223efe49..e95a938d63 100644
--- a/libs/pbd/event_loop.cc
+++ b/libs/pbd/event_loop.cc
@@ -20,6 +20,9 @@ EventLoop::set_event_loop_for_thread (EventLoop* loop)
 	thread_event_loop.set (loop, do_not_delete_the_loop_pointer); 
 }
 
+/** Called when a sigc::trackable that was connected to using the invalidator() macro
+ *  is destroyed.
+ */
 void* 
 EventLoop::invalidate_request (void* data)
 {