An Ag/alumina catalyst was prepared and tested for its HC(octane)-SCR activity using a gas mixture simulating exhaust gas from diesel and lean-bum vehicles. The catalyst showed excellent NO to N-2 activity, having a maximum conversion of approximately 90% at 450 degreesC and an average conversion of 66% in the temperature window 300-600 degreesC. During the high conversion of NO, formation of CO was recorded. A commercial oxidation ("cleanup") catalyst was placed after the Ag/alumina in order to remove the formed CO as well as unburnt hydrocarbons. This resulted. however, in a drastic drop of NO conversion, which was more pronounced when the distance was short between the two catalysts. When the distance between the two catalysts was increased to 33 mm (maximum value) the conversion was almost unaffected. This raised the question how nitrogen, once formed from nitrogen oxides, can seemingly disappear over the oxidizing catalyst at temperatures far too low to make this phenomenon thermodynamically feasible. In this study we propose that the HC-SCR reaction mechanism over Ag/alumina involves a gas-phase reaction between species created over the catalyst surface, and that this reaction is dependent on the residence time behind the catalyst. (C) 2003 Elsevier Inc. All rights reserved.