The effect of Ce3+, Co2+, and MoO42- inhibitor ions on the oxygen reduction reaction (ORR) was experimentally investigated on AA2024-T3 and selected castings containing intermetallic phases (i.e., Al-Cu-Mg and Al-Cu-Mn-Fe). All the inhibitors were found to reduce both the activation-controlled and mass-transfer-limited current densities associated with ORR. Molybdate ions were found to be the most effective cathodic inhibitors, followed by the cerium ions. Cobalt (II) ions were found to be effective, as well. The effect of inhibitors was modeled by considering three possible cases using a heterogeneous electrode model including an array of active electrodes embedded in an inactive matrix: (i) reduction of the size of electrochemically active sites, (ii) reduction of the spatial density or increase in the spacing between active sites, and (iii) formation of permeation barrier to O-2 diffusion. These models were found to be consistent with the experimental findings. (c) 2006 The Electrochemical Society.