The effect of hydrogen and carbon monoxide on the selective catalytic reduction of NOx with decane over a 2 wt% Ag/Al2O3 catalyst was investigated. Hydrogen promoted NOx conversion during the SCR reaction, in contrast to carbon monoxide, which had a slightly detrimental effect on catalyst activity, However, in situ UV-vis spectroscopy showed that both carbon monoxide and hydrogen promoted the formation of Ag clusters during the reaction. This, coupled with previous EXAFS experiments and UV-vis data on the rate of change of cluster formation, demonstrate that the enhanced activity from the addition of hydrogen to the NOx-SCR mix cannot be attributed exclusively to the formation of Ag clusters. It is proposed that hydrogen itself participates directly in the reaction mechanism. (c) 2005 Elsevier Inc. All rights reserved.