Using Auger electron spectroscopy and x-ray photoelectron spectroscopy we show that the unpinned GaAs is covered by a Ga203 layer with only small amounts of species containing arsenic. This oxide is formed by the consumption of GaAs during the unpinning treatment. We believe that it is this oxide that passivates the surface, allowing it to remain near flat-band for several hours in room air. The near absence of arsenic in the oxide, and the eventual repinning of the surface Fermi level supports the effective work function model of Fermi-level pinning. © 1988, The Electrochemical Society, Inc. All rights reserved.