In supercritical water at 400 degreesC and 0.5 g/cm(3) (37 MPa), benzaldehyde is decomposed into benzene and CO at a yield of 16% in 4 h; the latter is considered to be further converted into formic acid in the reaction condition. The decomposition competes against two types of disproportionation reactions. One is cross-disproportionation between benzaldehyde and formic acid, leading to the formation of benzyl alcohol and CO2. The other is self-disproportion of benzaldehyde where benzyl alcohol and benzoic acid are equally generated. The weight of the cross-disproportionation is larger than that of the self-disproportionation. As a result, the yield (6.6%) of benzyl alcohol is approximate to2.5 times as large as that of benzoic acid.