The effect of the addition of alkali metals on the catalytic activity of cobalt catalysts (Co/NaY, CO/SiO2, etc.) impregnated on inorganic oxides including NaY and SiO2 was studied using the gas-phase catalytic oxidation of benzyl alcohol. The partial oxidation activities, measured by the yield of benzaldehyde, of the supported Co catalysts were selectively promoted by adding alkali metal, with keeping the yield of CO2 low. The amount of added alkali metal under a constant amount of the impregnated cobalt had an optimal value for the catalytic activity of the benzaldehyde formation. The influences of the supporting method of Co, the inorganic oxide support, the amount of supported Co, and the kind of the added alkali metal on the partial oxidation activity were investigated to explore the supported Co catalysts for high catalytic activity. The sorpted oxygen species, rather than gaseous oxygen, were suggested to participate in the formation of benzaldehyde on the basis of a transient response experiment. The addition of alkali metal caused the increase in the amount of O-2 uptake of the prereduced Co/NaY catalysts. A correlation was observed between the amount of O-2 uptake and the amount of alkali metal added to the catalyst. The comparison between the diffuse-reflectance spectra of the Co/NaY catalysts with and without added alkali metal definitely revealed that the added alkali metal enhanced the affinity of the supported Co species toward oxygen to form the cobalt oxides (Co3O4 equivalent to Co2+(Co3+)(2)O-4). The cobalt oxides were suggested to be responsible for the partial oxidation of benzyl alcohol and the formation of benzaldehyde.