The promotional effect of tungsten in the CoMo/gamma-Al2O3 catalyst was studied for series of W-incorporated CoMo/gamma-Al2O3 catalysts with different content of tungsten. Two series of the catalysts were prepared by changing the impregnation order of cobalt and tungsten onto a base Mo/gamma-Al2O3 catalyst. Impregnation of tungsten was achieved under the condition that the pH of an aqueous impregnating solution of W anion was controlled to 9.5. The hydrodesulfurization (HDS) and hydrogenation (HYD) activities of the sulfided catalysts were evaluated by thiophene HDS and ethylene HYD reactions at atmospheric pressure, respectively. Low-temperature O-2 chemisorption at 195 K was conducted for the sulfided catalysts in order to determine the W-incorporation effects on the surface concentration of coordinatively unsaturated sites related to the catalytic activities. The dependence of catalytic activities on tungsten content showed initially an increase and subsequent decrease with increasing tungsten content. The maximum promotion of HDS and HYD activities occurred at a low content of tungsten corresponding to 0.025 in W/(W + Mo) atomic ratio regardless of the impregnation order of tungsten and cobalt. Oxygen uptake correlated well with catalytic activities. In general, the catalysts prepared by impregnating tungsten onto the CoMo/gamma-Al2O3 showed higher activities than the catalysts prepared by impregnating tungsten onto Mo/gamma-Al2O3 prior to impregnation of cobalt. (C) Academic Press, Inc.