While independent of the electrolyte, the *O adlayer present in region I seems to block the surface for the ORR completely. Increasing ORR activity is observed with the conversion of *O into *OH at the beginning of region IIc. The situation of O 2 interacting with an *OHcovered surface closely resembles that obtained for the ORR in an alkaline electrolyte. To explain the pronounced formation of H 2 O 2 under these conditions, Ramaswamy and Mukerjee suggested that the reaction occurs via an outer-sphere electron transfer reaction, with O 2 being solvated by an H 2 O solvation shell and separated from Ru by *OH. Our data fully agree with a picture where the adsorbing O 2 is separated by an *OH adlayer from the metal surface. We speculate that hydrogen from the *OH species is transferred to an O 2 species residing above the *OH in a second layer, forming a weakly adsorbed OOH species as in Eq. 1 (the ( * ) indicates a weakly adsorbing species). Another H is transferred to generate H 2 O 2 , where the source for the H can be another H from *OH (Eq. 2) or a proton from the electrolyte (Eq. 3).