The addition of copper to alumina-supported silver catalysts, by co-incipient wetness impregnation, increased the selectivity to nitrogen during the catalytic oxidation of ammonia at 250degreesC without significant decrease in activity relative to that of Ag/alumina alone. An increase in selectivity from 80 to circa 95% was observed at 100% ammonia conversion at the optimum Ag/Cu weight ratio (between 1:1 and 3:1). No increase in activity was observed for mechanical mixtures of Cu/alumina and Ag/alumina catalysts. Based on transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), low-energy ion-scattering spectroscopy, X-ray photoelectron spectroscopy, and Auger analysis, the catalysts consisted of monolayers of copper oxide on alumina upon which silver particles sat. TEM images and EDX analysis showed that silver particles, with a very large size distribution, existed on all catalysts. EDX also revealed the presence of very small (<1 nm) silver dispersed in copper everywhere on the alumina surface. Thus intimate contact between copper and silver on alumina-supported Cu-Ag catalysts existed. No indication of the formation of CuAl2O4 or of Cu-Ag phases was observed. The promotional effect of copper can be explained by a bifunctional mechanism in which the silver component mainly catalyzes ammonia oxidation to NO, the first step of this reaction, and the copper catalyzes the selective catalytic reduction of NO to nitrogen, thus reducing N2O formation on Silver.