A novel acoustic method has been used to characterize the zeta potential and potential of zero charge in the pores of sulfuric acid anodized aluminum as a function of pH and concentration of oxo-chromium(VI) compounds (those containing anions of chromate and dichromate). The results show that aqueous solutions of oxo-chromium(VI) at a concentration of 0.01 M substantially lower the zeta potential of the internal surface of aluminum oxide pores without irreversibly adsorbing as the Cr(lll) oxide. It remains as the adsorbed but otherwise soluble oxo-Cr(VI) moiety. This explains in part the corrosion inhibition of anodized aluminum by oxo-Cr(VI) compounds, since such a change in zeta potential discourages adsorption by Cl-.