OpenEars expects to be launched on mainThread and to handle its own multithreading, but I wouldn’t automatically expect a problem with launching it on a secondary thread. Certain operations will always be returned to mainThread (notifications sent to OpenEarsEventsObserver and delegate callbacks of OpenEarsEventsObserver will always go to mainThread, I think that audio playing in FliteController must be initiated mainThread IIRC, maybe there are some other examples of things which expect to initiate from or return to mainThread). But, if all you are doing is running LanguageModelGenerator and you are running it on a background thread, my guess is that the issue is in the actual threading, because LanguageModelGenerator works as a series of single-threaded activities so it should be possible to background. Are you using blocks, NSTimer or NSThread?