Four carboxylic acids with a number of a-hydrogen atoms ranging from three to zero were tested in the selective hydrogenation to aldehyde. The acids used were acetic, propanoic, isobutyric, and pivalic acid. The oxides of iron, vanadium, zirconium, and titanium were used as catalysts. It was found that by decreasing the number of a-hydrogen atoms the selectivity to the aldehyde increased, while the formation of the main by-product, ketone, was suppressed. It is suggested that this is due to the fact that the ketonisation proceeds via a ketene-like intermediate, the formation of which needs the presence of a-hydrogen. Furthermore, the reactions to aldehyde and ketone seem to be in competition with each other.