Supporting CuO on a Al2O3-coated cordierite honeycomb yields a good catalyst (CuO/HC-Al) for selective catalytic reduction (SCR) of NO with NH3 at 350 - 500 degrees C. SO2 has complex effects on the catalyst's activity. It significantly promotes the SCR activity through conversion of CuO to CuSO4, however, when a certain amount of CuO is converted, it slightly decreases the SCR activity through competitive adsorption with NH3. This competitive adsorption reduces the amount of NH3 adsorbed on the catalyst surface, especially on the sites highly active to the SCR. It also prevents transformation of CuO to CuSO4 and as a result, the catalysts subjected to pre-sulfation and in situ sulfation show different SCR behaviors.