A microkinetic model of the oxygen reduction reaction (ORR) on Pt(111) under a gaseous H-2 and O-2 atmosphere is used to predict and explain which compositions of H-2 and O-2 lead to the fastest rate of water formation for temperatures between 600 and 900 K. For a stoichiometric (2:1) mixture of H-2 and O-2 the rate-determing step is found to transition from O* hydrogenation to O-2* dissociation over this same temperature range. These results are explained in terms of the temperature dependence of the surface coverages of O* and H* and are shown to be consistent with kinetic models aimed at understanding the ORR under electrochemical conditions.