To understand the activity and stability of the electrode under reaction conditions, it is helpful to know the potential dependent adlayer composition of the electrode, which can at least approximately be inferred from studies in O 2free electrolytes. Such aspects were, however, only discussed more vividly around the time when the work by Inoue et al. appeared or later. Therefore, the mechanistic insights provided in the earlier study on ORR on Ru(0001) of Inoue et al., may not be applicable or accurate to the present day.