The reactivity of copper-on-alumina catalysts in the reduction of NO by NH3 + O-2 (SCR reaction) and oxidation of NH3 to N-2 (NH(3)ox reaction) as a function of the copper loading shows that (i) the specific rate of the SCR reaction per mole of copper decreases with increasing copper loading, whereas the NH(3)ox reaction rate is nearly independent of the copper loading and (ii) the rate of the NH(3)ox reaction is significantly higher in the absence of NO than that of the same reaction occurring during the SCR reaction. The catalysts were characterized by electron spin resonance CESR), X-ray diffraction analysis, and UV-visible diffuse reflectance. The nature of the NO and NH3 adsorbed species was studied using ESR and infrared (IR) spectroscopies, and IR studies of the reactive coadsorption of NO and NH3 were carried out. The results obtained indicate that the above catalytic effects are due to the formation of nitrate species which block the reactivity of copper sites. The transformation of NO to N-2 via the intermediate formation of ammonium nitrate was shown, but the data suggest that this is only a secondary pathway of reaction responsible for the formation of N2O as a byproduct. The nature of active copper species on the surface and the presence of competitive pathways of transformation of NO to N-2 are also discussed.