The formation of surface intermediates and their reactivities with so me reactant gases during the selective catalytic reduction of NO with ethylene (C2H4-SCR) over Co-ZSM-5 was investigated systematically by in situ diffused reflectance infrared Fourier transformed spectroscopy. Some new IR bands relevant to the surface intermediates formed in the title reaction were identified. The results show that formate species, formed during the partial oxidation of ethylene over Co-ZSM-5, is reactive towards nitrates to yield formyl nitro compound, an intermediate critical for the formation of cyanide and isocyanate species by further reacting with nitrates. Kinetics analysis suggests that the cyanide and isocyanate at different sites vary in reactivity towards nitrates. The Co2+-CN and Al3+-NCO species subsequently react with nitrates for final production of N-2, CO2 and H2O, however, the -CN bound to Bronsted and Lewis acid sites show poor reactivity towards a flow of NO + O-2/He. On the basis of these investigations, a proposed reaction mechanism explains the formation and roles of all intermediates detected by IR spectroscopy in this study. (C) 2009 Elsevier B.V. All rights reserved.