Catalysts based on cobalt-iridium supported on zirconium-doped mesoporous silica, with a Si/Zr molar ratio of 5, a cobalt loading of 16 wt.% and Co/Ir molar ratios of 10, 20, 30 and 60 were prepared by impregnation, characterised and then tested in the selective catalytic reduction of NO with ammonia in an excess of oxygen. These catalysts are active in the targeted catalytic reaction showing high conversions of NO at low reaction temperatures with a low yield of N2O. The incorporation of irridium increases the dispersion of cobalt, thus improving the catalytic performance. These catalysts maintain their activities when water is added to the feed on stream, but are less active with the addition of 100 ppm of SO2. (c) 2005 Elsevier B.V. All rights reserved.