An In2O3/Al2O3 catalyst shows high activity for the selective catalytic reduction of NO with propene in the presence of oxygen. The presence of SO2 in feed gas suppressed the catalytic activity dramatically at high temperatures; however it was enhanced in the low temperature range of 473573 K. In TPD and FT-IR studies, the formation of sulfate species on the surface of the catalyst caused an inhibition of NOx adsorption sites, and the absorbance ability of NO was suppressed by the presence Of SO2, and the amount of ad-NO3- species decreased obviously. This leads to a decrease of catalytic activity at higher temperatures. However, addition Of SO2 enhanced the formation of carboxylate and formate species, which can explain the promotional effect Of SO2 at low temperature, because active C3H6 (partially oxidized C3H6) is crucial at low temperature. (c) 2006 Elsevier B.V. All rights reserved.