The selective catalytic reduction of NO in excess Oz with C3H6 as reductant was studied over a Rh/alumina catalyst. The effect of SO2 on the extent of NO reduction and product formation was extensively examined. TPD and FT-IR studies using different feeds aimed at the identification of species adsorbed on alumina and Rh/alumina samples. Experiments were also carried out in a differential fixed-bed reactor to estimate apparent orders of reaction and activation energies. The rate-controlling step of the proposed reaction mechanism for the NO reduction is the reaction of Rh-NCO and Rh-NO+ for an SO2-free feed. In the presence of SO2 in the feed the NO and C3H6 oxidation over the catalyst were inhibited and a less complicated reaction mechanism is proposed, based on the reaction of Rh-NOf and partially oxidized C3H6