The selective catalytic reduction (SCR) of NO to N-2 by NH3 over TiO2, ZrO2, and Al2O3 was studied at 90 degreesC using a feed stream that was simulated to represent the effluent from a coal combustion boiler. The effects of SO2 on the reduction of NO were studied. It is interesting that the SCR reaction was interrupted when SO2 was absent from the feed gas. This indicated that SO2 participated in the reduction process of NO at low temperature. A mechanism for this low-temperature SCR process was proposed, which was based on the reaction results and studies of NO, NH3, and SO2 temperature-programmed desorption (TPD) profiles under different experimental conditions. Among the three catalysts, ZrO2 and TiO2 had higher activities than Al2O3 The selectity to N-2 over the different catalysts changed in the sequence of ZrO2 > TiO2 > Al2O3 The activities of these three catalysts decayed with reaction time as a result of the accumulation of ammonium sulfate and nitrate salts plugged in the pores of the catalysts. However, the deactivated catalysts could be easily regenerated by washing with water.