In the HER region, where the surface is usually covered by a dense *H adlayer, the adsorbed *H does not have a significant impact on the ORR, as evident from the continuing and even increasing ORR current in this regime. On the other hand, the *H species also do not seem to play a significant role in the formation of H 2 O 2 , based on the low yields obtained in that region. Most simply, this can be explained by continuous desorption of H 2 and re-adsorption of hydrogen from H + , which leaves a chance also for O 2 molecules to directly interact with the Ru(0001) surface in temporarily existing vacancies in the *H-adlayer. Comparing the ORR activity in the HER region of both electrolytes, we observed a lower activity in H 2 SO 4 than in HClO 4 , even though *HSO 4 can be excluded in this region. We suggest that the lower activity in H 2 SO 4 is caused by electrolyte effects, which has to be addressed in a separate work.