In the positive-going scan, in the peak at potentials more positive than 0.0 V, adsorbed hydrogen is displaced by *OH in HClO 4 or *HSO 4 in H 2 SO 4 to form anodic molecular hydrogen. At potentials more positive than that peak, the processes from the negativegoing scan occur in reverse order. Considering the different peak structures in the positivegoing scan in this potential region indicates that the potentials for the processes are shifted and that the adlayer composition at a given potential is somewhat different in both scan directions. ORR -general aspects: Figures and show the ORR-CVs recorded in O 2 -saturated electrolytes at 20 mV s -1 . The ORR-CVs were recorded during a sequence of cycles, and subsequent cycles did not show measurable differences, illustrating that the electrodes are stable under these conditions. This conclusion is in line with that derived from the STM images shown in Fig. , which did not resolve any significant changes after the electrocatalytic measurement. Figures and show the partial currents for the formation of H 2 O 2 (H 2 O 2 -CV), deduced from the currents at the collector electrode. Contributions from the oxidation of H 2 formed on the working electrode at high overpotentials can be excluded (see Section S1 in the SI). This finding agrees with previous works, which showed that the oxidation of H 2 is little efficient on the oxidized Pt collector electrode set at 1.2 V. The corresponding H 2 O 2 yields are depicted in Figs. and. Note that the values obtained very close to the onset potential must be considered cautiously since small variations in the currents on the working and collector electrodes can lead to a strong over-or underestimation of the H 2 O 2 yield. Therefore, these data are not included in the plot. The evaluation of the underlying data and their error ranges are described in Section S1 in the SI.Additional pseudo-staircase voltammograms (SCVs), which showed current at selected potentials that were very similar to those obtained in the ORR-CVs, and a comparison of ORR-CVs recorded at different scan rates, illustrate that at most potentials, the measured ORR currents are not affected by slower adsorption/desorption processes (see Section S2 of the SI). ORR in HClO 4 : Starting with the ORR-CV for Ru(0001) recorded in 0.1 M HClO 4 , the onset potential for the ORR in the negativegoing scan is located at slightly more positive values than that of the surface reduction processes observed in the BCV. This is shown more clearly in Section 3 of the SI,where the low current region of the ORR-CV is plotted on top of the BCV. Hence, close to the onset potential, the ORR proceeds on the largely *O-covered surface, possibly containing small amounts of *OH. Continuing with the negative scan, the ORR current increases steadily in the range between the onset and about 0.2 V. From this potential, the current is almost constant until the end of region IIc and throughout region IIIc. Apparently, the substantial changes in the ad-layer composition in this potential range are hardly reflected in the ORR current. Finally, the ORR current increases again in the HER region IVc. This latter increase in the ORR current also contains contributions from the HER. The contribution is, however, small, based on the current measured in the BCV at the lower potential limit (150 µA cm -2 ). Note that the ORR on Ru(0001) is not transport-limited, which is consistent with our conclusion that the dominant contribution to the increase in current in this potential range originates from the ORR kinetics.