Oxide layers on polycrystalline GaAs thin films were grown by thermal oxidation at 10 torr. In depath profiles of the oxide layers were obtained by in situ Ar+ ion etching in X-ray photoelectron studies at different stages of sputter etching. These studies reveal a pile up of elemental As at the interface between the GaAs film and the oxide layer. The observ-ed compositional variation in the oxide layer at different depths is related to the selective sputtering of As 0 to As. Some of these results are presented. © 1992 SPIE. All rights reserved.