Oxide layers have been formed on (110) and (100) GaAs wafers by exposure to a high kinetic energy beam of atomic O and characterized using x‐ray photoemission spectroscopy (with Ar+ion sputter profiling) and Raman spectroscopy. Photoemission shows the reacted layer, ∼500 A˚ thick, to be uniform in composition and fully oxidized. Raman spectroscopy shows that the substrate is not appreciably disordered during oxidation and in some cases no free‐elemental As is present at the oxide‐substrate interface at a detectable level.