The present work investigates the catalytic properties of the Ni, Co and Co-Ni supported activated carbon (AC) catalysts for the selective oxidation of carbon monoxide in an excess hydrogen gas. The results shows that Co/AC and Co-Ni/AC exhibit a higher catalytic activity than Ni/AC, meanwhile, Co-Ni/AC gives a high CO conversion in a wide reaction temperature region of 130-150 degrees C, the CO conversion can reach up to above 99.5%. The X-ray power diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterizations show that Co3O4 is the main cobalt oxide phase in Co/AC and Co-Ni/AC catalysts. The XRD and temperature-programmed reduction (TPR) results indicate that Co3O4 species of Co-Ni/AC exhibits a higher dispersion than that on Co/AC. Based on the XPS results, the function of nickel oxide in Co-Ni/AC is to enrich Co3O4 species, and decrease the electronic cloud density of cobalt. Therefore, we conclude that Co3O4 species is an important catalytic active center, the higher activity of Co-Ni/AC in comparison with Co/AC is attributed to the higher dispersion of active Co3O4 species and the lower electronic cloud density of cobalt on the surface of Co-Ni/AC. (c) 2005 Elsevier B.V. All rights reserved.