The catalytic activity of Ir/WO3/SiO2 for the selective reduction of NO with CO in the presence of excess O-2 was significantly increased by the presence of H2O, whereas NO reduction hardly occurred in the absence of H2O. Exposure of the reaction gas with H2O to Ir/WO3/SiO2 at high temperature led to the stabilization and/or regeneration of reduced Ir sites, as suggested by X-ray diffraction and Raman spectroscopy. Temperature-programmed reduction by H-2 and a combination of scanning transmission electron microscopy and elemental analysis showed the presence of a strong Ir-W oxide interaction. Based on the activity and characterization results, we concluded that the promoting effect of coexisting H2O is accounted for by the formation of the catalytically active reduced Ir sites, by H-2 produced in situ via water gas shift reaction, interacting strongly with W oxide dispersed on SiO2 via migration of It to W oxide or vice versa. (C) 2010 Elsevier Inc. All rights reserved.