Mechanistic and kinetic experiments were carried out on the NOx selective catalytic reduction (SCR) by C3H6 in excess O-2 over In/Al2O3 in the presence and absence Of SO2 and H2O. The In loadings in the catalyst varied in the range of 1-4%. The 2% In/Al2O3 exhibited the highest activity. The performance of the aforementioned catalyst was measured using different pretreatment procedures. Pretreatment with O-2 resulted in a more active catalyst than pretreatment with H-2 or He either when the feed contained H2O and SO2 or when it did not. The presence of H2O (0-10%) enhanced the catalyst activity, while SO2 (0-500 ppm) acted as a poison by decreasing the NOx conversion as compared to the SO2-free experiments. Kinetic and temperature programmed desorption (TPD) studies showed that a different reaction mechanism for the NOx reduction over In/Al2O3 applied when H2O and SO2 were present or absent in the feed. In the former case the reduction proceeds via the reaction of a reaction intermediate (CxHyOzN), formed by the interaction Of C3H6 and NO2, with other activated NOx species. In the latter case, NOx species adsorbed on In react directly with CxHyOz to form nitrogen and combustion products. (C) 2003 Elsevier B.V. All rights reserved.