Metalorganic chemical vapor phase deposition of GaN on (100) GaAs has been studied using mass spectroscopy. With increasing substrate temperature, the amount of decomposed trimethylgallium (TMGa) was observed to increase exponentially with a characteristic energy of 1.5 eV. The presence of NH3 was found to suppress the production of CH3 in the gas phase. This implies that CH3 of TMGa reacts with the hydrogen atom of NH3, forming CH4 as a main gas product. Studies of nitrogen evaporation from the growth surface when TMGa flow was off lead to the conclusion that increased growth rate could result in decreased background electron concentration due to nitrogen vacancy. The presence of NH3 significantly promotes the decomposition of TMGa. Desorption of excess Ga atoms from the growth surface at low NH3 flow rates takes place as suggested by the increased ratio of peak intensity of Ga (m/e = 69) to that of DMGa ((CH3)2Ga, m/e = 99) with decreasing NH3 flow rate.