The formation of nitrate and NO2 adspecies over Cu/MFI and copper-on-alumina catalysts and their role in the mechanism of reaction is discussed on the basis of FT-IR results and catalytic tests in unsteady-state conditions. Three specific cases are discussed : (i) reduction of NO by propane/O-2 over Cu/MFI, (ii) conversion of NO by NH3/O-2 over copper-on-alumina catalysts and (iii) oxygen-promoted reduction of NO in the absence of reductants over Cu/MFI. The formation of nitrate species leads to self-deactivation, but Cu2+-NO2 like adspecies are suggested to be a key intermediate in the reduction of NO to N-2 in all three cases examined.