Structural properties: The quality of the electrode surface after the preparation (see Experimental Section) and the stability after the electrocatalytic characterization were verified by scanning tunneling microscopy (STM) measurements under UHV conditions. A representative STM image of the as-prepared surface in Fig. shows extended atomically flat terraces with widths of several 100 nm, separated by monoatomic high and largely straight steps. Images recorded after the electrocatalytic experiments in O 2 -saturated 0.1 M HClO 4 and 0.5 M H 2 SO 4 (Figs. and) illustrate that the electrode surface closely resembles the asprepared surface, without any indications of restructuring. Electrochemical properties: The electrochemical properties of Ru(0001) were determined from cyclic voltammograms (CVs), recorded in bare supporting 0.1 M HClO 4 and 0.5 M H 2 SO 4 electrolytes, which are shown in Figs. and (denoted as base CVs -BCVs). The BCVs resemble those reported previously, and a detailed description of the processes within different potential regions or peaks can be found there. Most important for the present work is that certain potential regions are characterized by different adlayer compositions. This is illustrated in Figs. and, where the labels I to IV denote the different regions and the suffixes c and a denote the cathodic (negative-going) and anodic (positive-going) scans. More elaborate Starting with the negative-going scan, in HClO 4 , OH* partially desorbs, leaving about 0.5 monolayers (ML) of *OH on the surface at 0.1 V. At the end of region III, the remaining *OH adlayer is replaced by *H at the peak at 0.0 V. In H 2 SO 4 , the *OH is first increasingly replaced by adsorbed bisulfate (*HSO 4 ). Note that replacing *OH with *HSO 4 can yield a net current of zero (when both species are exchanged one to one). Therefore, other techniques than cyclic voltammetry are required to elucidate this process. The mixed *OH (0.3 ML) / *HSO4 adlayer is then replaced by an *H adlayer in the peak at -0.2 V. Region IV is finally dominated by the HER, where the surface is covered by an *H adlayer in both electrolytes.