Comparing with the similar-type measurements on the supported Pt/C catalyst, we find similar trends on a qualitative scale, with some differences in the potential region of the onset of COad oxidation and in particular in the range of OHad formation and Pt surface oxidation. In the latter region the H2 signal is clearly lower than in the low-potential region (COad blocking) for the supported Pt/C catalyst. Apparently, there is either still some residual HOR activity on the Pt/C catalyst under these conditions or, more likely, there is a measurable H2 consumption by chemical reaction of dissolved H2 with Pt surface oxides over the Pt/C catalyst with its larger electrochemically active surface area, while this is essentially inhibited on the pc-Pt electrode. Effects caused by the different electrochemically active surface areas as well as other structural differences between the two electrode materials, in particular in the type and content of defects, are considered as reasons for the differences in the HOR behavior between Pt/C catalysts and pc-Pt.