Selective catalytic reduction (SCR) of NO with ammonia in an excess of oxygen was carried out over cobalt-platinum impregnated on a zirconium containing mesoporous silica (Si/Zr molar ratio of 5) with a 12 wt.% of Cobalt and Co/Pt molar ratios of 10, 20 and 30. These catalysts were compared with two catalysts containing 12 wt.% of Co and 1.5 wt.% of Pt, respectively. The presence of platinum improves the reducibility of Co(III) to Co(II) and at the same time the presence of cobalt ions impedes the reduction of Pt(II) to Pt(0). This gives rise to a synergetic effect, especially in the case of the catalyst with a Co/Pt molar ratio of 30, exhibiting a good performance in the SCR of NO by ammonia, even when water or SO2 are added to the feed on stream, and maintaining high conversion and selectivity at moderately low temperatures (250 degreesC). In fact, this catalyst maintains a conversion of 84% at 300 degreesC in the presence of water during 24 h. (C) 2004 Elsevier B.V. All rights reserved.