CoOx catalysts (Co 1 and 3 wt.%) were prepared by incipient-wetness impregnation of the supports, Al2O3 and Al2O3 (80 wt.%)-BaO (20 wt.%), and calcined at 500 and 800degreesC. The samples were characterized by X-ray diffraction (XRD) and BET techniques. H-2-temperature programmed reduction (TPR) and UV-Vis diffuse reflectance spectroscopy (DRS) spectra were recorded with the aim to identify the different Co species formed. The presence of Co2+ species tetrahedrally and octahedrally coordinated appears strongly influenced by the nature of the support and the cobalt content. Co3O4 particles were detected after calcination at 500 degreesC, a successive treatment at 800 degreesC promotes the dispersion Of Co3O4 clusters in the network of alumina with formation of CoAl2O4 spinels. The catalysts were tested in the selective catalytic reduction (SCR) of NO with C3H6 in the presence of excess O-2, using a reactant mixture containing NO 1000 ppm, C3H6 1000 ppm, O-2 5%. The effectiveness of the cobalt catalysts in the SCR process depends strongly on the type of support, metal loading and calcination temperature. The presence of barium oxide in the alumina network is effective in the stabilization on the surface of dispersed Co2+ ions which are active and selective for NO reduction in excess Of O-2. Clusters of cobalt oxide, present as Co3O4 in the alumina samples calcined at 500degreesC, are active mainly for C3H6 combustion. (C) 2002 Elsevier Science B.V All rights reserved.