With further increasing potential the Faradaic current and also the H2 consumption decrease again. Nevertheless, at 1.1 V there is still a clearly measurable current. On the other hand, from the fact that the H2 signal reaches a similar value as obtained at low potentials (within the precision of determining the H2 background and thus the actual H2 signal), where due to COad blocking the reaction is completely inhibited, we conclude that also at this potential (1.1 V) the HOR is essentially fully inhibited. Hence, the presence of surface oxides completely inhibits the HOR at 1.1 V, and the remaining Faradaic current is due to ongoing surface oxidation, which may be enhanced by chemical reduction of surface oxides by reaction with molecular H2. In the negative-going scan and subsequent cycles the Faradaic current as well as the H2 signal are identical to the traces shown in Fig. . This can easily be explained by the fact that after oxidation of the pre-adsorbed CO adlayer the state of the Pt surface is identical to that in the HOR experiment shown in Fig. .