A series of Co/SiO2 catalysts containing 10-30 wt.% Co were prepared by wet impregnation and tested in the Fischer Tropsch reaction using a simulated syngas (CO/H-2) mixture similar to that obtained in biomass gasification. The catalysts characterization included N-2 adsorption, temperature programmed reduction (TPR), X-ray photoelectron spectra (XPS) and transmission electron microscopy (TEM) techniques. The reaction was carried out in a stainless steel fixed bed reactor at 300 degrees C and 1 Mpa. In general, activity increased almost linearly with increasing Co-loading, reaching maximum at about 20 wt.% Co and then levelled off. These results correlate with that derived from XPS. The Co/Si surface atomic ratios increased almost linearly up to about 20 wt.% Co due to gradual surface coverage by Co species. At higher loadings, cobalt form large crystal aggregates. The selectivity to C-8-C-9 hydrocarbons increases whereas the C14+ hydrocarbon follows an opposite trend upon increasing Co-loading. It was also observed that the selectivity to liquid fraction is a function of the Co crystallite size, that is longer hydrocarbons chains are formed on the smaller cobalt crystallites. (C) 2009 Elsevier B.V. All rights reserved.