Herein we report that biomimetic analogues of cytochrome c oxidase (CcO) couple reduction of O-2 to oxidation of a single-Ru(NH3)(6)(2+) under steady-state catalytic turnover. electron carrier , Higher Ru-II concentrations favor the 4-electron vs 2-electron O-2 reduction pathway. Our data indicate that the capacity of electrode-adsorbed Fe-only porphyrins to catalyze reduction of O-2 to H2O is due to high availability of electrons and is eliminated under the biologically relevant slow electron delivery.