It is found that the addition of Mo to Rh/SiO2 (Mo-Rh/SiO2) promoted alcohol formation in hydroformylation of propylene and ethylene, at the same time it drastically enhanced the activity of hydroformylation and hydrogenation. It is suggested that H-2 adsorption sites, which are not inhibited by CO adsorption, are present on Mo-Rh/SiO2. The sites can cause high H-2 activation, and it can be related to the additive effect of Mo in hydroformylation reactions.