An acoustic test method has shown that Ce(III) cations adsorb reversibly on anodized Al so as to increase the zeta potential of the oxide in the porous anodized layer. Adsorption of anionic chromate species on the identically prepared surfaces decreases the zeta potential. The role played by Ce(III) in improving the corrosion resistance of anodized Al differs significantly from that provided by hexavalent chromium oxo-anions.