We revisited the ORR on Ru(0001) in O 2saturated 0.1 M HClO 4 and 0.5 M H 2 SO 4 to gain a better atomic-scale mechanistic understanding of the complex, potential-dependent reaction behavior, extending the potential range into the HER region. Using the results of our previous studies and assuming that the potential-dependent adlayer composition is similar in the absence and presence of O 2 , we could demonstrate that the potentialdependent activity for both the ORR and the formation of H 2 O 2 is closely related to the potential-dependent changes in the composition and structure of the adlayer. In both electrolytes, the onset of the ORR coincides with the onset of the partial reduction of the *O-dominated adlayer.