It is shown that the Fischer-Tropsch synthesis in the presence of Co-xMn/SiO2 cobalt catalysts with a manganese content of 0.5-2 wt % is characterized by high activity and C5+ hydrocarbon selectivity. The optimum concentration of manganese as a promoter is 1 wt %. The influence of manganese on the state of the surface of the catalysts in oxide and reduced forms is studied by a number of physicochemical methods. The formation of solid solutions of cobalt and manganese oxides of the xCo(3)O(4)-(1-x)Mn3O4 type is confirmed. The relationship between the promoting effect of manganese and the structure of active sites and the character of adsorption of H-2 and CO on the surface of the catalysts is ascertained.