The chemistry of the GaAs oxides fabricated by local anodic oxidation (LAO) using an atomic force microscope is studied by means of spatially resolved Auger electron spectroscopy. Oxide structures with nanometric size grown at different bias are investigated. We found that the LAO oxide is essentially composed of gallium oxides. The chemical composition of the LAO oxide does not depend on the applied bias. (C) 2004 Elsevier B.V. All rights reserved.