Hydrogenations of two model compounds (acetone, propanal) have been studied with Pt/SiO2 catalysts, promoter-free or promoted by Ga, Ge or Fe compounds. It appeared that hydrogenation of acetone is faster than that of propanal, but the promotion effect is more pronounced with the aldehyde than with the ketone. This is in compliance with earlier work, A tentative explanation of the observed phenomena is suggested, based on the theoretical analysis by Delbecq and Sautet.