In summary, we show that O 2 adsorption has a very different response to applied potential during the ORR on Pt(111) and Au(111). The strong potential dependence found at Au(111) is specifically lost at Pt(111) and we attribute this contrast to the inherently different reactivity between the two metals. Reductive charging of the non-reactive Au(111) shows more of a capacitor-like behavior that drives the transition from a preferred physisorbed, to a superoxo-like and finally a peroxo-like O 2 * at the surface. As a result, the effect of the