The chemical composition of an irreversibly adsorbed layer of arsenic on Pt(111) in sulfuric acid solution has been studied by X-ray photoelectron spectroscopy (XPS). From the chemical shift of the As 3d level, a change in the valence state from As(0) to As(III) with positive-going electrode potential is deduced, the total amount of As on the surface remaining constant. The As coverage derived from XPS is around 0.33 ML. which is in agreement with the charge under the current peak in the cyclic voltammogram. From the coadsorption of anions, accompanying the valence transition of As, As(III) is assumed to exist as As(OH)(3) on the surface. (C) 2004 Elsevier Ltd. All rights reserved.