In HClO 4 , the rate continuously increases with more negative potentials until saturating before the HER region. In the HER region, the current increases again, with ORR and HER occurring simultaneously. In the reverse scan, the rate is almost reversible. In contrast, in H 2 SO 4 , ORR is only active in regions where no blocking *HSO 4 is on the surface, i.e., in the negative-going scan between the onset of the ORR and -0.2 V and in the HER region. No reaction occurs in the positive-going scan at potentials above the HER. Significant H 2 O 2 yields were observed in the potential region where surface *O is transformed into *OH (region II) in both electrolytes. Its formation is rationalized by an outer-sphere reaction mechanism, where hydrogen is transferred from the OH adlayer to the O 2 residing in the electrolyte above the electrode. In combination, these results demonstrate the enormous effects adsorbed species can have on the ORR reaction characteristics in general. Furthermore, the results of this work can serve as a perfect basis and reference system for further theoretical studies.