For potentials more positive than peak A1, the COOR current density declines with increasing potential, which is more pronounced for the Pt-1 than for the Pt-3 electrode. This potential region is often denoted as the mass transport-limited region. In the present case, however, under enforced mass transport, kinetic limitations seem to be active as well, leading to the observed decay in COOR rate with increasing potential. The presence of kinetic limitations is also consistent with the lower COOR current on the Pt-3 electrode in this potential region as compared to the Pt-1 electrode (for similar flow rates). For each of the two electrodes, the current densities exhibit the same potential dependence for different UPLs. This is also true for the Pt-1 electrode, which tends to restructure under these conditions (see Fig. ). Apparently, the processes responsible for the decay in COOR rate with increasing potential are fully reversible between subsequent cycles.