The chemical states of the surface of the oxygen ion conducting solid electrolyte La0.9Sr0.1Ga0.85Mg0.15O3-delta (LSGM 1015) as prepared by solid-state synthesis was analyzed by X-ray photoelectron spectroscopy. It was found that adventitious carbon did not interact with any of the constituent elements of LSGM 1015. Ga and La were found to exist in trivalent states. But, due to ionic bombardment presence of Mg could not be detected in the electrolyte surface. (c) 2007 Elsevier Ltd. All rights reserved.