In the positive-going scan, the amount of H 2 O 2 formation decreases reversibly towards more positive potentials for the same reasons as the increase in the negative-going scan in this region. Interestingly, at the potential where the small minimum in the H 2 O 2 formation current was observed in the negative-going scan, now a similar size maximum appears, which is also shifted to more positive potentials (0.1 V). Similar to the behavior in the negative-going scan, the ORR current is almost constant in this potential region. In this region, the BCV also does not show any noteworthy features. Hence, possible adlayer changes are too small to result in detectable currents in the BCV or do not involve a charge transfer. At potentials more positive than 0.2 V, a similar sized peak as in the negative-going scan is observed, whose maximum is also about at 0.3 V. Hence, the H 2 O 2 formation increases again with increasing *OH coverage in region IIa. The fact that the maximum of this peak is about in the middle of region II indicates that it results from a combination of two effects: (i) an increasing ORR activity with more cathodic potentials and (ii) an optimum composition of this *OH/*O adlayer. Note that the peak reaches far into the potential range of region Ia, consisting of an *OH and *O adlayer, where the ORR currents are negligible (Fig. ).This effect is possibly caused by the large amounts of H 2 O 2 formed in region IIa, where part of it is retained in the electrolyte channel between the working and collector electrodes. This effect is less pronounced for CVs recorded at lower scan rates, as shown in Fig. . steadily decreases in the negative-going scan, from approximately 60% close to the onset of the ORR to a few percent at around 0.2 V and remains this low also in the HER region. In the positive-going scan, the H 2 O 2 yield increases almost reversibly in region IIa, with increasing *OH coverage, also reaching approximately 60% close to the onset potential. In total, H 2 O 2 is formed mainly in regions where a significant amount of *OH is present on the surface (> 0.5 ML *OH in region II). Small amounts are also formed when the surface is covered by *H, i.e., in the HER region IV. This conclusion will be corroborated by the ORR characteristics in 0.5 M H 2 SO 4 discussed next. ORR in H 2 SO 4 : For the ORR on Ru(0001), in O 2 -saturated 0.5 M H 2 SO 4 the ORR-CV in Fig. is more complex. Similar to the ORR in 0.1 M HClO 4 , the onset potential in the negative-going scan is located at slightly more positive values than that of the surface reduction processes observed in the BCV (see Section S4).Hence, the ORR starts on the largely *O-covered surface in region I, as in HClO 4 . This current develops into a distinct peak in region IIc in the potential range, where *O is increasingly replaced by *OH. With decreasing potential, the latter is partly replaced by adsorbed bisulfates (*HSO 4 ), forming a mixed *OH/*HSO 4 adlayer, with the amount of *OH decreasing and that of *HSO 4 increasing with more negative potentials in region IIIc. In this region, the electrode is essentially inactive. The current increases again at the onset of the HER region IV. The onset coincides with the peak at -0.2 V in the BCV and is therefore likely caused by the same surface process, i.e., exchange of the mixed *OH/*HSO 4 adlayer by *H. Significant ORR currents are obtained once the surface is mostly covered by *H, but the overall current is smaller than in O 2 -saturated HClO 4 .