Abstract Vapor phase hydroformylation of ethene and propene was studied with Co/SiO2 catalysts. A selectivity of oxygenates of Co/SiO2 derived from Co2(CO)8 was markedly improved by the addition of alkaline earth oxides such as Ca, Sr, and Ba, although these additives decreased the activity of the catalyst. They also promoted the formation of propanoic acid and aldol condensates of propanal in ethene hydroformylation, and decreased the fraction of n-oxygenates in the total oxygenates in propene hydroformylation. EXAFS studies revealed that the active site for hydroformylation is a highly dispersed cobalt metal. Alkaline earth oxides promote to CO insertion to surface alkyl species by enhancing the interaction between CO and Co metal.