The adsorption and surface reactions of trimethylgallium and tertiarybutylarsine on GaAs(100) surfaces have been investigated by Fourier transform infrared spectroscopy. Adsorbed methyl groups resulting from the dissociative chemisorption of trimethylgallium on GaAs(100) are shown to form As-H and CH2 species on the surface. The CH2 groups are stable on the surface at temperatures as high as 550 °C. The surface coverage is low (∼0.2% of a monolayer) and is reduced by the presence of hydrogen on the surface. This dehydrogenation of surface methyl groups could be a possible route to carbon incorporation in GaAs grown by atomic layer epitaxy. Tertiarybutylarsine is shown to decompose primarily by homolysis to form a tertiary butylgroup and AsH2. At temperatures below 400°C on trimethylgallium dosed surfaces, the decomposition products appear to cause the hydrogenation of methylene groups remaining from prior surface dosing with trimethylaallium. At high temperatures, the tertiarybutyl radical appears to undergo dehydrogenation reactions to an unsaturated species which is stable on the surface. In contrast, the dehydrogenation does not appear to occur on surfaces treated with tertiarybutylarsine. The data for trimethylgallium and tertiarybutylarsine support the general assertion that surface As-H species play a critical role in the removal of hydrocarbon species from the growth surface.