Pretreatment conditions for the activation of Ir/WO3-SiO2 for the selective catalytic reduction of NO by CO in the presence of excess O-2 were studied. Sequential treatment involving calcination in the presence of O-2 and H2O followed by reduction and then re-oxidation under mild conditions was found to effectively activate Ir/WO3-SiO2. Temperature-programmed desorption during calcination, X-ray diffraction, and temperature-programmed reduction by H-2 revealed that calcination was necessary for oxidative removal of the NH3 ligands from the iridium precursor, that reduction produced metallic iridium and partially reduced tungsten oxide, and that re-oxidation produced tungsten oxide with low reducibility. Transmission electron microscopy revealed that It was supported on finely dispersed tungsten oxide and that the iridium particle size after the sequential activation was 1-1.5 nm. (C) 2008 Elsevier B.V. All rights reserved.