The retarding potential electron beam technique has been used to study the photovoltage spectrum of Ar-bombarded GaAs (100) surfaces. Clean surfaces and those with absorbed O2, NO, NH3, H2S, SO2, and CO2 were investigated. The spectral region above the band-gap energy shows structure which changes in position and intensity in ways characteristic of the adsorbed gas. Similar structure is seen on the GaAs (100) surface prior to any Ar bombardment. Heating and cooling the sample do not result in appreciable changes to the photovoltage spectra, but heating does cause a transient loss of photovoltaic activity. Several possibilities for the origin of the photovoltage structure are discussed, including surface states and electron–phonon interaction.