The oxygen reduction reaction (ORR) taking place at the cathode of a fuel cell is considered to play a major role in the energy loss in such systems. Desirably, the reduction of O 2 proceeds to form H 2 O. However, an indirect path that involves the formation of hydrogen peroxide (H 2 O 2 ), which can further react to H 2 O, is equally possible. Pt is the most active pure elementary metal catalyst for the ORR, and, therefore, also the most intensively studied one for this reaction, including a vast number of studies on well-defined single-crystal model electrodes and Pt-based materials. Much less is known about the reaction pathways on metals that bind reaction intermediates stronger than Pt. In particular, for these metals, it must be considered that the interaction with the electrolyte and anions in the electrolyte also plays a significant role, which in turn can affect the potential dependent reaction characteristics. One metal that exhibits a strong interaction with electrolytes is Ru.