The formation of dihydrogen and benzoic acid was observed from water vapor and benzaldehyde over Cr3+-promoted ZrO2, ZrO2 and Cr2O3 catalysts. Cr3+-promoted ZrO2 was the most active catalyst for this reaction and an equimolecular amount of dihydrogen and benzoic acid was obtained. Dihydrogen and the corresponding carboxylic acids were also obtained from the reaction of 3-phenoxybenzaldehyde or 1-heptanal with water. It is suggested that a surface carboxylate species formed by aldehyde adsorption is an intermediate of these reactions.