Oxygen reduction on Cu in near-neutral 5% NaCl and its subsequent inhibition by Cr(VI) and Ce(III) compounds has been examined using stationary and rotating Cu electrodes. The results show that Cr(VI) is a highly efficient inhibitor not matched by comparable concentrations of Ce(III) in its effect. Results suggest (i) bulk reduction of Cr(VI) to a three-dimensional inhibiting film is not necessary to explain the role of Cr(VI); (ii) Cr(VI) inhibition of oxygen reduction on Cu is irreversible as shown previously by others; and (iii) inhibition of oxygen reduction by Cr(VI) at dispersed Cu cathodes in Al2024-T3 cannot explain the 100-fold increase in corrosion resistance typically observed for Al 2024-T3.