In Fig. we present cyclic voltammograms (CVs) and mass spectrometric scans recorded in H2saturated 0.5 M H2SO4, which show the removal of H2 with the onset of the HOR. The HOR current exhibits a broad plateau region from ca. 0.0 to 0.7 V, which is defined by the H2 mass transport limitations. This is reflected also by the H2 signal, which remains constant up to about 0.7 V. At that point the formation of adsorbed OH and subsequently of surface oxides sets in, leading to a decrease of the Faradaic current and also of the H2 consumption of H2. The latter is reflected by an increasing H2 signal. At 1.1 V the Faradaic current is close to zero, but has not fully disappeared. The remaining current can result either from Pt oxidation or from ongoing HOR. In addition, also reduction of electrochemically generated PtO by the dissolved H2 may occur. From these mass spectrometric data we cannot decide so far, however, whether there are still contributions from the HOR or not, since the partial pressure of H2 for the case of complete inhibition of the HOR is not available under the present experimental conditions. The situation is different for the experiment presented in Fig. , which shows the positive-going scan, recorded on a pc-Pt electrode initially pre-saturated with adsorbed CO, and the subsequent cycling of the COad-free electrode after electrooxidation of COad (COad stripping), all measured in H2-saturated electrolyte. Saturated CO adlayers are known to completely inhibit the HOR. This is evident also from the absence of a measurable oxidation current at low potentials in Fig. (upper panel), where on the COad-free electrode the HOR proceeds already in the mass transport limited regime (Fig. ). Therefore, the mass spectrometric signal recorded on the COad-blocked electrode indicates the H2 partial pressure signal obtained in the absence of any measurable H2 consumption. At about 0.45 V the Faradaic current starts to increase, and also the H2 signal starts to drop (Fig. , upper and middle panel). At the same time we find a slight increase of the CO2 signal (Fig. , bottom panel). This can be explained by the onset of COad oxidation, which is clearly seen from the CO2 formation. The removal of COad enables also H2 oxidation on the largely COad-blocked Pt surface. At about 0.6 V these trends increase rapidly, leading to a distinct peak in the CV and to a broad minimum in the mass spectrometric H2 signal. At the same time, CO2 formation and thus COad oxidation increase rapidly and pass through a sharp maximum. This minimum reflects the transport limited HOR on a surface that is partly free from adsorbed species such as COad or OHad. It is important to note that the transport limited HOR current is already reached when there is still significant CO2 formation/COad oxidation and therefore significant amounts of COad on the surface. Due to the combination of COad blocking at low potentials and Pt oxide blocking at high potentials the potential range of active HOR is much narrower than in the HOR on the COad-free Pt electrode (Fig. ), both in the Faradaic and in the mass spectrometric H2 signal (Fig. ).