The hydrogenolysis of ethyl acetate on Raney copper catalysts has been previously suggested to proceed via dissociative adsorption followed by hydrogenation of adsorbed fragments. As part of a programme to study ester hydrogenolysis, the hydrogenation of acetaldehyde, which could be suggested to involve similar adsorbed fragments, has been examined. The kinetics of the reactions of ester and aldehyde with hydrogen are found to be different, and isotopic labelling techniques show that the intermediates are probably different. Acetaldehyde hydrogenation appears to involve adsorption through oxygen followed by attachment of adsorbed atomic hydrogen to the oxygen. Aldehyde hydrogenation is much faster than ester hydrogenolysis, by a factor of 6000, under identical conditions.