Yes chapters do work in QuickTime Player, it happily shows ones created with ffmpeg. Those are in fact in a text track, not mebx. I've seen references to text tracks on the net, however I've had no luck creating one and getting a working movie file out. There doesn't seem to be an adapter (besides AVAssetWriterInputCaptionAdaptor, which also didn't work) to write to the text track. Any chance you could show some code?