Acetic acid has been used as a model compound in the selective hydrogenation of aliphatic acids, which contain alpha-hydrogen atoms, to their corresponding aldehydes. In contrast to what the literature predicts, it appeared to be possible to produce acetaldehyde directly from acetic acid. The appropriate catalyst consists of an oxide with an intermediate metal-oxygen bond strength. Addition of platinum to the catalyst enhances selectivity and activity. A mechanism is proposed, based on the involvement of lattice oxygen (viz., a Mars and Van Krevelen mechanism) and the spill-over of activated hydrogen from the platinum to the oxide. The most important side reaction is the formation of acetone from two molecules of acetic acid (ketonization), but this reaction is suppressed completely by the addition of platinum to the catalyst.