The interaction of DeNO(x) components comprising NO, NO2, propane, O-2 and H-2 with a selected Ag/Al2O3 catalyst was studied by in situ FTIR spectroscopy at a fixed temperature where a promoting effect of H-2 admixture on the catalytic NOx reduction has been reported to occur. The significant enhancement of nitrato and acetate ad-species from NO and propane, respectively, could be confirmed. New findings are the relative inertness of these species for further reaction progress. Instead, nitrite, nitrito, and nitro species are reactive, and the H-2 co-fed favours the formation of those species. Nevertheless, the way H-2 interferes with the kind of species formed includes the promotion of oxidative reaction steps evidenced by different effect of H-2 on the interaction of the catalyst with NO/O-2 and NO2, respectively. During NO2/H-2 adsorption adsorbed NOx species and Ag+ ions at the surface are reduced. This prevents abundant formation of stable nitrato species and favour the formation of largely unstable but reactive nitro and nitrito species. Otherwise, NO2 is able to oxidize pre-reduced Ag/Al2O3. Furthermore, indications were found for minor propene formation from propane in the presence of hydrogen. (C) 2004 Elsevier B.V. All rights reserved.