The catalytic reduction of NOx under oxygen rich conditions in the presence of mixed H-2 + CO reductant feeds occurs much more efficiently on Pd/Al2O3 than Pt/Al2O3 catalysts [Appl. Catal. B 35 (2002) 269]. In effect, CO promotes the former while poisoning the latter. Here in situ DRIFTS was used to investigate details of the reaction mechanisms on both these catalysts. Under various reaction conditions, nitrate, carbonate and bicarbonate species were observed in both cases. However, formate and isocyanate (NCO) species were observed only on the Pd/Al2O3 catalyst. Formate is shown to be spectator, whereas in contrast NCO shows significant reactivity. Although pre-adsorbed NCO is consumed in flowing NO + O-2, its reactivity is much greater in H-2 + NO +O-2. We propose that hydrolysis of NCO results in the formation of NH3 species, whose production is ultimately responsible for improved NOx reduction by Pd/Al2O3 in the presence of H-2 + CO. (C) 2003 Elsevier B.V. All rights reserved.