The effect of iridium dispersion on the activity of Ir/SiO2 for the selective reduction of NO with CO in the presence of O-2 and SO2 has been investigated. The intrinsic activity, expressed in terms of turnover frequency (TOF), increases with decreasing iridium dispersion, indicating that NO reduction with CO over Ir/SiO2 is a structure-sensitive reaction. Characterization of Ir/SiO2 using TPO, XRD and FT-IR spectroscopy following CO adsorption shows that larger iridium crystallites are more difficult to be oxidized and much easier to be reduced under reaction conditions, resulting in the formation of stable iridium metal sites on which NO reduction occurs. (c) 2006 Elsevier B.V. All rights reserved.