The present work uses X-ray photoelectron spectroscopy (XPS) to compare the chemical composition of the outer surface of porous and sealed anodic films on commercially pure Al and on Al-Cu, Al-Si-Mg, and Al-Mg alloy substrates. The study also considers some inner surfaces, obtained by mechanical abrasion and argon ion bombardment, in the bulk of porous and sealed anodic films on the Al-Mg alloy. The anodic films seem to be constituted mainly by pseudobohemite, with a uniform distribution of sulfate ions throughout the coating. Especially high O/Al ratio values have been determined on the outer surface of hydrothermally sealed films, which is interpreted as being due to the extra fixing of water and hydroxyl groups on the pseudobohemite. (c) 2007 The Electrochemical Society.