We have used a combination of pulsed molecular beam and time-resolved mass spectrometry to study the kinetics of the pyrolysis of trimethylgallium on GaAs(100) surfaces. We found that CH3 is the major reaction product. Two CH3 desorption channels were observed, with activation energies 37.9±1.6 and 45.0±1.4 kcal/mole. An arsine ambient significantly accelerates the CH3 desorption, but no CH4 was observed. A model for the reaction of trimethylgallium on the GaAs(100) surface is proposed.