The selective catalytic reduction of NO with ethylene was investigated and compared over fresh CoH-ZSM5, H-2-reduced (450 degrees C) CoH-ZSM-5 and H-ZSM-5. The dispersion of cobalt species for fresh and H-2-reduced CoH-ZSM-5 was studied by UV-vis and FTIR spectra. Combined with DRIFTS results of ad-species and reaction experiments, the active sites of CoH-ZSM-5 for the C2H4-SCR were identified and the catalytic functions of active cobalt species and support were also clarified. Notably, the formation of -CN or -NCO species was heavily dependent on the type of nitrates. Co3O4 particles contributed to the oxidation of NO to bidentate nitrates. The latter species were crucial for the formation of -NCO. Whereas exchanged Co2+ ions facilitated the oxidation of NO to monodentate and/or bridged nitrates which participated in the formation of -CN species.