Two Th-added Co/SiO2 catalysts were prepared by pore volume impregnation of 6 wt% Co3O4/SiO2 With either aqueous or ethanol solutions. Smaller ThO2 crystallites were obtained for the ethanol-prepared Th/Co3O4/SiO2. The reduction behavior of cobalt oxide was modified by aqueous impregnation. Furthermore, Th addition was found to be promotional in the case of ethanol impregnation, or detrimental in the case of aqueous impregnation, for the CO oxidation activity of Co3O4/SiO2. Th addition has no effect on the extent of reduction of cobalt phase, but (1) reduces the crystallite size of cobalt by hindering the sintering of cobalt metal; (2) increases the amount of hydrogen desorbed (150 or 190%), CO adsorbed (3 or 4 times), CO2 evolved (2.6 or 4 times), and carbon deposited; (3) enhances the activity for CO disproportionation; (4) induces new CO adsorption modes, subcarbonyl-like species (Co(CO)(x), x > 1) and multifold CO ((Co)(x)CO, x greater than or equal to 2); (5) decreases methanation activity; and (6) enhances the activity and the selectivity for higher hydrocarbons and olefins for Fischer-Tropsch synthesis. A roughening of the cobalt surface, which consists of steps and kinks induced by the decorated ThO2 moieties around the interface area, is the proposed origin for the new adsorption and CO disproportionation sites. The more significant effects of Th addition when ethanol was employed as impregnation solvent instead of water is attributed to a more extensive Co-ThO2 interface formed due to a better dispersed Th phase.