The selective catalytic reduction of NO+NO2 (NOx) at low temperature (180-230 degreesC) with ammonia has been investigated with copper-nickel and vanadium oxides supported on titania and alumina monoliths. The influence of the operating temperature, as well as NH3/NOx and NO/NO2 inlet ratios has been studied. High NOx conversions were obtained at operating conditions similar to those used in industrial scale units with all the catalysts. Reaction temperature, ammonia and nitrogen dioxide inlet concentration increased the N2O formation with the copper-nickel catalysts,while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNO(x) activity, with no detectable ammonia slip and a low N2O formation when NH3/NOx inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH3/O-2, NO2/NH3/O-2 and NO/NO2/NH3/O-2 feed mixtures indicated that the presence of NO2 as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N2O formation. When NO was also present, N2O formation was not observed. (C) 2000 Elsevier Science B.V. All rights reserved.