The equilibrium (reversible) potential for oxygen reduction reaction (ORR) on Pt electrodes was evaluated using small cluster models and DFT calculations. For the five elementary reactions, O-2 -> OOH, OOH -> O + H2O2 OOH -> HOOH, 0 -> OH, and OH -> H2O, the estimated equilibrium potentials were 0.41, 1.99, -0.35, 1.36, and -0.01 V. Comparing to the standard electrode potential for 02 reduction, 1.23 V, the OOH and H2O formations contribute to the overpotential, and HOOH formation does not prevail.