Low pressure (∼10−5 Torr) studies of the adsorption and decomposition of tertiarybutyl arsine (t-BAs) on three different GaAs(100) surfaces (Ga-rich, H-atom treated and high temperature t-BAs treated) are presented. Mass spectrometric studies indicate that the primary gas-phase products of t-BAs decomposition on GaAs(100) are the tertiary-butyl radical and incomplete arsenic hydrides (predominantly AsH2). Infrared spectroscopic studies show that on H-atom treated and high-temperature t-BAs treated surfaces, t-BAs decomposes leaving behind very small quantities of alkyl groups. On Ga-rich surfaces however, significant amounts of alkyl products are observed. The surface species resulting from the adsorption of trimethyl gallium (TMG) are shown to be different at room temperature and high temperature.