Photoemission has been used to study the effects of chemisorption of nitric oxide (NO) on the valence-band and Ga and As 3d core levels of (110)-GaAs wafers. A parallel sequence of experiments using O2 has also been carried out. For exposures of 104–107 L at 300 K, NO dissociates and reacts with the surface more slowly than does O2. This stage involves defects and shows no photoenhancement. Above 107 L oxidation occurs, at least as fast for NO as for O2, and electronic excitation of the substrate by visible/near-ultraviolet irradiation during large NO exposures is found to enhance the formation of As–O species. Little N is detected on the surface after oxidation by NO. A model for the differences in oxidation behavior of NO and O2 is proposed.