The selective catalytic reduction (SCR) of nitric oxide by propene under lean-bum conditions over Ir/Al2O3 and Ir/SiO2 was studied. Irrespective of the support, Ir supported catalysts were found to be active in the reduction of NO above 673 K, Ir/SiO2 being the most active. For both catalysts, the activity in the reduction of NO into N-2 was shown to be strongly enhanced after their exposure at 873 K under the reaction mixture. In spite of varying initial Ir dispersions depending on the metal content and the support, Ir sintered after activation in the reaction mixture, which led to the same final Ir dispersion. An in situ FTIR study of both catalysts at various temperatures under the reaction mixture and under CO or NO used separately was carried out. It was shown that, under the reaction mixture, the surface of initially fully oxidised Ir particles progressively reduced with increasing temperatures into partially reduced Ir8+ surface species allowing the adsorption of both CO and NO (vCO at 2070-2050 cm(-1) and vNO at 1870 cm(-1)). Additional species (formate, acetate, nitrate) formed only on the Al2O3 support surface but these species are thought not to participate into the NO reduction. (C) 2003 Elsevier B.V. All rights reserved.