The activity in terms of conversion of carbon monoxide was determined for Co/SiO2 catalysts in CO hydrogenation over a reaction time of 120 h. The catalysts were prepared from nitrate (N) and carbonyl (CO) precursors. The conversion decreased rapidly during the first five hours, and thereafter moderately at a rate related to dispersion, i.e, the higher the dispersion the higher the rate of decrease. The active sites were blocked by wax and coke formed in the reaction, although some agglomeration of particles probably took place on the Co(CO)/SiO2 catalysts. More carbon was accumulated on Co(CO)/SiO2 than on Co(N)/SiO2 during the reaction suggesting a need for frequent regeneration. The reduction-oxidation-reduction treatments indicated, however, that the regenerability of the Co(CO)/SiO2 in terms of hydrogen uptake is poor, although the amounts adsorbed still remained higher than those for Co(N)/SiO2.