The effects of Co loading and calcination temperatures on the catalytic activity of Co/Al2O3 for selective catalytic reduction (SCR) of NO with ethylene in excess oxygen were investigated. Co/Al2O3 showed high and low activities when calcined at high (800 A degrees C) and low (350 A degrees C) temperatures, respectively. The formation and dispersion of cobalt species for catalysts calcined at 350 and 800 A degrees C as well as for Al2O3 were studied by XRD, UV-vis and FTIR spectra. Combined with DRIFTS results of ad-species and reaction experiments, it allowed us to correlate the catalytic activity with active sites of Co/Al2O3, and the catalytic functions of active cobalt species and support were clarified. Co3O4 species contributed to the oxidation of NO to various nitrates and of C2H4 to reactive formate species, even in the absence of O-2, whereas the side reaction of ethylene combustion occurred simultaneously when excess oxygen was present. Tetrahedral Co2+ ions in CoAl2O4, which acted as the active sites, were responsible for the reaction between formate and nitrate species to form organic nitro compound.