The promoting effects of magnesium, vanadium and cerium oxides on cobalt and ruthenium/carbon catalysts in the hydrogenation of carbon monoxide have been compared. The reducibility of the promoted and unpromoted catalysts was investigated by temperature-programmed reduction. In all cases the added oxide promoter enhanced the specific activity and the selectivity for alkenes and long chain hydrocarbons. These results are discussed on the basis of a general mechanism of promotion.