Benzhydrol and benzoin, which have two benzene subunits, a secondary hydroxy group, and no hydrogen atom on the β-position of the hydroxy group, were treated in supercritical water in the absence of an oxidizing agent or a catalyst. Water played a key role in the product distributions in these reactions. Each reaction proceeded smoothly to give larger amounts of oxidation products, benzophenone (3) and benzil (6), as well as smaller amounts of reduction products, diphenylmethane (4) and benzyl phenyl ketone (7), respectively, in the presence of water, while the ratios of 3:4 and 6:7 were almost 1:1 in both cases in the absence of water. The best yield of benzophenone (63%) was achieved by the reaction of benzhydrol at 460°C for 180 min in 0.35 g mL-1 water density in the SUS 316 reactor. Hydrogen gas evolution was observed in the reaction of benzhydrol in a quartz tubular reactor.