The Pt-catalyzed reduction of NO by CO exhibits strong electrochemical promotion (EP) by backspillover Na supplied from beta " alumina under appropriate conditions of temperature, gas composition, and catalyst potential, At high CO partial pressures (P-CO) the EP effect is attenuated by CO island formation and limited availability of chemisorbed NO. At low P-CO, the CO + O reaction is limited by the low coverage of CO and the EP effect is again relatively small. In the intermediate P-CO regime, Na pumping strongly accelerates the reaction : rate gains of up to 1300% for N-2 production, relative to the unpromoted rate are observed, This can be understood in terms of Na-induced dissociation of chemisorbed molecular NO, which is thought to be the reaction initiating step. The overall kinetic behavior and the dependence of N-2/N2O selectivity on catalyst potential are in good accord with this hypothesis. Thus the EP system also provides direct information about the nature of the reaction mechanism under conditions of elevated pressure.