Laser ionization mass spectrometry has been used to study the deposition of gallium from trimethylgallium with and without AsH3. The apparent Arrhenius activation energy for the production of gas-phase methyl radicals from trimethylgallium is measured to be 28 ± 2 kcal/mol in the presence of AsH3, about the same value as measured in the absence of AsH3. At a substrate temperature of 1150 K where gallium desorption is substantial, addition of AsH3 is found to increase methyl radical yield but drastically decrease gallium atom desorption. A mechanism is presented to describe the deposition of GaAs at low pressures under single gas-surface collision conditions.