This article presents a method for the observation of ammonia dissociation during the gas source molecular-beam epitaxial (MBE) growth of group III nitrides. The mass spectrometer, present in most molecular beam epitaxy machines, is used to detect H-2, a biproduct of ammonia dissociation. A direct line of sight, from the mass spectrometer to the substrate, is not required. The technique is used to examine several aspects of the MBE growth of AIN, GaN, and GaInN. Aluminum is approximately six times more efficient in causing ammonia dissociation than is gallium, which in turn is similarly more efficient than indium. During the growth of GaInN, transient effects in ammonia dissociation are observed following indium shutter actuations, indicative of indium surface accumulation. (c) 2005 American Vacuum Society.