We suggest that the differences in the reaction characteristics can be explained by the structural differences between the well-defined Ru(0001) on the one hand and Ru(pc) electrodes with their different surface sites and also much higher defect densities on the other hand. Based on the Sabatier principle, the abundance of different adsorption sites with different Ru-O / Ru-OH adsorption energies will lead to different reaction rates on different sites. Furthermore, different adsorption energies will also shift the regions of stable anion adsorption, which is crucial, e.g., for anion adsorptioninduced reaction inhibition effects. Therefore, it is not surprising that i) the ORR activities differ for Ru(0001) and Ru(pc) and that ii) potential dependent inhibition effects, e.g., due to the presence of *HSO 4 , are not well resolved or even hardly visible on the Ru(pc) sample.