The oxygen reduction reaction (ORR) was examined on copper, platinum, and glassy carbon electrodes, with regard to its inhibition by exposure of the electrode to chromate ion (Cr-VI) in NaCl solution. All three electrode materials exhibited a mass transport limited current for the ORR at sufficiently negative potentials, but this current was strongly inhibited in the presence of Cr-VI. Inhibition persisted in Cr-VI-free solution after the electrode was rinsed thoroughly, indicating that Cr-VI formed an irreversibly adsorbed inhibiting layer. A reduction peak observed in Cr-VI solution had an area of 1.1-2.5 mC/cm(2), and the area varied little with Cr-VI concentration, electrode material, and potential in the range of +0.2 to -0.6 V vs. Ag/AgCl. This reduction peak is attributed to Cr-III formation, and corresponds to formation of approximately a monolayer of Cr-III oxyhydroxide. Once formed, this Cr-III monolayer inhibits both O-2 reduction and further reduction of Cr-VI. The onset of monolayer formation at about +0.25 V vs. Ag/AgCl is the same as the potential of the onset of ORR inhbition in dilute Cr-VI. The monolayer also decreases the electron transfer rate to ferrocene and Ru(NH3)(6)(+3), which are known to be outer sphere redox systems that do not require adsorption to the electrode surface. The results indicate that the adsorbed Cr-III film formed by Cr-VI reduction is a powerful inhibitor of oxygen reduction, due both to occupation of active chemisorption sites and to inhibition of electron transfer. In the context of corrosion protection, Cr-VI acts as a "site-directed'' irreversible inhibitor which migrates to active sites for the ORR, then is reduced to Cr-III, and forms a permanent inhibiting monolayer.