Selective catalytic reduction of NOx with propene has been investigated over SnO2/Al2O3 catalysts prepared by impregnation and single step sol-gel methods. Catalyst preparation method exerts significant influence on the catalytic performance, and SnO2/Al2O3 prepared by the sol-gel method is more active than the impregnated catalyst. Moreover, its resistance against water is higher. The high surface area and the strong interaction between SnO2 and Al2O3 may be responsible for the high reduction activity of sol-gel SnO2/Al2O3 catalyst. The presence of oxygen strongly favours the NOx reduction process. FT-IR results have revealed that the role of oxygen is to contribute to the formation of formate, acetate and nitrate species, all of which are reactive intermediates for NOx reduction. (C) 2010 Elsevier B.V. All rights reserved.