The physicochemical properties of 10% Co/silica-alumina cat.&& prepared by impregnation were studied by TPR, thermoanalysis and carbon monoxide adsorption. The temperature at which the catalyst was pretreated was found to have a great influence on its adsorption capacity for carbon monoxide. Both reduced and unreduced samples calcined in air at relatively high temperatures adsorbed carbon monoxide, but the reduced ones adsorbed much larger amounts. The adsorption capacity tended to increase with increasing pretreatment temperature. The character of carbon monoxide TPD profiles and the proportions of the individual adsorption forms were found to depend on tbe pretreatment temperature and the degree of cobalt reduction. Carbon monoxide was found to be adsorbed in three forms on species originating in cobalt-support interactions, metallic cobalt and on CosO,. Cakination in air was found to cause a decrease of both cobalt reduction and the activity in hydrocarbon synthesis. While the yield of gaseous products remained constant, that of the liquid ones decreased substantially with increasing pretreatment temperature, reaching a maximum value when the degree of cobalt reduction was ca. 40-50%. The average carbon number decreased with increasing cobalt reduction. The catalytic properties of Co/silica-alumina are more similar to those of Co/SiOz than of Co/A&O,, but Co/silicaalumina catalysts are less efficient in polymerization tban either Co/SiO, or Co/Al,Ox. The hypothesis, suggesting that the adsorption centres of weakly bonded carbon monoxide were involved in the production of liquid hydrocarbons and that the cobalt oxide species act dire&y in this synthesis, was confirmed.