From our previous study of Ru(0001) in O 2 free electrolytes, it is evident that in the potential region from the onset of the ORR to the HER region, the surface is always at least partly, if not fully covered by adsorbates, as illustrated in Fig. . Therefore, we suggest that in addition to a reaction pathway involving the adsorption of O 2 directly on the Ru(0001) surface and thus a direct interaction between Ru and O 2 , there must be another reaction pathway allowing for ORR also on adsorbate covered surfaces / sites. The first pathway would be limited to the potential regions where *OH is partly removed to form water in HClO 4 . Such situations can be found near the transition from region II to III and in region III (or vice versa). The second pathway, in contrast, must be possible in potential regions where the adlayer is essentially closed, e.g., at the more positive potentials of region II or in the HER region IV.