The contribution of ionic reactions to phenol oxidation has been investigated in subcritical and supercritical water. From the experiments conducted at 24.7 MPa, it was found that the phenol conversions at 360 degrees C were slightly higher than that at 400 degrees C. We calculated the concentration of phenolate anion at 24.7 MPa using semiempirical model reported by Sue et al. (2002). The calculated concentration of phenolate anion at 360 degrees C is about seventh order of magnitude higher than that at 400 degrees C because of the high ion product of water at 360 degrees C. The electron transfer (ET) reaction (C6H5O- + O-2 = C6H5O- + O-2(-)) was investigated as one of the reasons for the enhancement of phenol oxidation in subcritical water. We estimated the equilibrium constant of the ET reaction using density functional theory calculations, which suggests that the ET reaction should be taken into consideration in the region of high ion product of water. We also discussed the dimerization reaction as consumption reactions of phenoxy radical and other reactions involving inorganic ion species such as O-2(-), OH- and HO2-. When phenoxy radical and O-2(-) are produced effectively, these reactions could be more favorable in subcritical water than in supercritical water. From these discussions, phenol oxidation in subcritical water could be enhanced by the ionic reaction mechanism because of a high ion product of water.