A study of the premature room temperature gas‐phase reactions involved in the growth of using , , and was undertaken, using a specially designed mass spectrometer sampling system on a conventional, low pressure OMVPE reactor. It was shown that the reaction of the mixture could be described by reversible bimolecular reaction kinetics within the limits of experimental error, and evidence for an adduct is presented. The equilibrium constant for this system was determined as 30 torr; the rates of forward and reverse reaction were 5 (torr‐s)−1 and 0.15 s−1, respectively. The implications of the reaction of this organometallic source with arsine on the growth mechanism is discussed in this paper.