Ab initio molecular orbital techniques have been used to study the elementary gas phase reactions involved in the metallo-organic vapor phase epitaxy of GaAs from trimethylgallane and arsine. These reactions were identified by Tirtowidjojo and Pollard (J. Cryst. Growth 1988, 93, 108) using a comprehensive mathematical model that incorporates multicomponent heat and mass transport, fluid flow from an impinging jet system, and reaction kinetics in the gas phase and at the growing surface. Calculations show that the abstraction of a hydrogen atom from arsine by either a hydrogen atom or methyl radical is significantly exothermic and that the activation barriers for these processes are extremely low, confirming the transient absorption spectroscopic results of Gaskill et al. (J. Cryst. Growth 1988, 93, 127) for the latter process. New calculations of the energy required to break a Ga-C bond in trimethylgallane and dimethylgallane confirm that relatively high temperatures are required for the homogeneous decomposition of these molecules, emphasizing the importance of surface catalysis at lower temperatures.