This paper reports the results of an experimental study involving Fischer-Tropsch (FT) synthesis on modified Co/Si-2-Al2O3 catalyst. The objectives are to increase the selectivity of C-5(+) liquid hydrocarbons (HC) and to decrease the formation of gaseous hydrocarbons during the reaction. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and powder X-ray diffraction (XRD) techniques. The XPS results indicated the formation of a new surface species of cobalt complex on the surface of the catalyst.