The hysteresis in the potential region between 0.1 V and the onset of the ORR is likely related to the different adlayer compositions on Ru(0001) in both scan directions shown previously. We suggest that the hysteresis is similar in O 2 -free electrolyte (Fig. ) and most likely also in O 2 -containing electrolyte. Different from the negative-going scan, the *H adlayer extends in the positive-going scan up to the peak centered at 0.15 V (Fig. , region IIIa, BCV), where anodic H 2 is formed upon the adsorption of *OH in the BCV. Preliminary differential electrochemical mass spectrometry (DEMS) measurements, which allow monitoring the formation of volatile species in the electrolyte, suggest that anodic H 2 also forms in the presence of O 2 during the ORR (see Section ).Continuing the positive-going scan, the ORR ceases at the end of region IIa, with the formation of a dense *O adlayer. , does not follow the the ORR-CV in Fig. . Instead, the H 2 O 2 formation starts in the negative-going scan with the onset of the ORR at 0.6 V and continues in a broad peak, where the approximately 1 ML *O adlayer transforms into a mixed *O/*OH adlayer (see Fig. ). The H 2 O 2 formation is almost suppressed at about 0.15 V (end of region IIc), where the surface is covered by about 0.5 ML *OH. The negligible activity remains throughout region IIIc (see Fig. ). Apparently, the *OH coverage is too low to support H 2 O 2 formation despite considerable ORR activity in this region.