A series of Co-CeO2/SiO2 catalysts at cobalt iso-content (25 wt%) with various concentrations in cerium (4.5, 8.9, 21.5 and 38.2 wt%) and a 25 wt% Co/SiO2 system have been characterized and tested in Fischer-Tropsch synthesis for the production of high molecular weight hydrocarbons (waxes). The effects of cerium oxide addition on the cobalt particle size, on the reducibility of the cobalt and on the catalytic performance (activity and selectivity) of a Co/SiO2 catalyst have been studied. The catalysts have been prepared with an original method combining the coprecipitation of the precursors of cobalt and cerium by oxalic acid with the hydrolysis and condensation of the tetraethoxysilane. The crystallite size measurements (TEM) of Co3O4 and CeO2, which are the only structures detected by XRD and XPS in oxidized samples, indicated that the presence of cerium had a dispersing effect on the cobalt. It was also found that the ceria was well dispersed on the silica support. The reduction experiments (TPR and XPS) under hydrogen at 673 K showed that the reduction rate of ceria supported on SiO2 was similar to that of ceria with high specific surface area. The cobalt reduction degree decreased with the addition of cerium, even in low concentration. It was however equivalent for the Co/CeO2, Co-Ce(38.2 wt%)/SiO2 and Co/SiO2 catalysts allowing to conclude that the extent of cobalt reduction at 773 K was not very much affected by the reduction state of ceria. The catalytic activities (TOF) (P=2 MPa, T=493 K, H-2/CO=2/1) of Co-CeO2/SiO2 and Co/SiO2 catalysts were similar. With the addition of cerium significant changes in hydrocarbon selectivities were observed: decrease in C2+ hydrocarbon selectivity and production of gasoline to the detriment of the C22+ fraction (chain growth probability (alpha) from 0.82 to 0.87 for Co-CeO2/SiO2 catalysts and 0.92 for the unpromoted catalyst). This modification was attributed to a smaller cobalt particle size, to the specific adsorption/desorption properties of H-2 and CO on Co-CeO2/SiO2 catalysts and also to the possible presence of formate species, which could be formed on the partially reduced ceria pointed out by H-2 and CO TPD studies. The hydrogenation of these species could explain the additional methane production in the Fischer-Tropsch synthesis under pressure.