It was found that the catalytic activity and selectivity of Pd for the reduction of NO by CO at temperatures of 600 to 750 K can be markedly and reversibly affected by depositing a polycrystalline Pd film on 8 mol% Y2O3-stabilized-ZrO2 (YSZ), an O2- conductor, and applying external negative currents or potentials between the catalyst and a Au counter electrode. The increase in the rate of reduction of NO is typically more than 700 times larger than the rate of removal of O2- from the catalyst via negative current application, while the CO2 and N-2 formation rates can be respectively doubled and tripled. The promotional effect is explained by an enhanced dissociation of NO on the Pd surface which is moderated by a stronger adsorption of CO relative to NO. The N2O + CO reaction was also studied and exhibited a twofold enhancement of the rate, in the most favorable cases, as negative or positive potentials or currents were applied. It was concluded that this reaction is not an important route to the formation of N-2 during the NO + CO reaction under these conditions.