The selective catalytic reduction (SCR) of nitric oxide by propene over Ir/Al2O3 under lean-bum conditions (1000 vpm NO, 2000 vpm C3H6, 500 vpm CO, 10 vol.% O-2) was studied. The activity was shown to be strongly enhanced after exposure of the catalyst at 600 degreesC under the reaction mixture, irrespective of the oxidising or reducing pre-treatment. Simultaneously, the Ir dispersion decreased from 78 to 10%. The influence of each component of the reaction mixture on the activation process was examined. The presence of both CO and O-2 was found to be necessary to activate Ir/Al2O3 while NO would not be. In situ FT-IR results revealed that initially fully oxidised Ir particles partially reduced in the feed to form Ir-0 reduced surface sites (vCO at 2060 cm(-1)) which adsorbed CO up to 350-400 degreesC. The activation under reactants was related to the formation of these sites. The presence of reduced (or partially reduced) Ir sites, possibly siting at the surface of IrO2 particles and stabilised by CO adsorption, was proposed to be responsible for the SCR activity.