The effect of pretreatment atmosphere, hydrogen or carbon monoxide, on the activity and selectivity of the Rh4(CO)12/SiO2 and Co4(CO)12/SiO2 catalysts in CO hydrogenation was determined. The catalysts pretreated under hydrogen exhibited a higher degree of decomposition of the original cluster, higher hydrogen uptake and lower carbon content than those decomposed under CO. Therefore, some of the active sites of the catalysts were encapsulated by carbon, and in the case of Co4(CO)12 poorly reducible cobalt silicates were probably also formed. The activity of the hydrogen-treated Rh4(CO)12/SiO2 was fairly similar to that of the CO-treated catalyst, but significantly fewer oxygenates were formed. Thus, the carbon monoxide treatment appeared to facilitate the formation of oxygenates. Probably the original structure of rhodium carbonyl was better retained, and thereby some of the rhodium particles remained more dispersed under CO than under hydrogen. In the case of Co4(CO)12/SiO2 catalysts, however, no benefit was observed in conjunction with CO pretreatment either in activity or selectivity. Most likely the CO treatment resulted in the formation of unreactive carbonaceous species which permanently blocked most of the active sites.