The pH of aqueous phenol solutions at subcritical and supercritical conditions was measured for the first time at temperatures from 296.4 to 673.4 K and pressures from 29.8 to 34.9 MPa by using a flow-through electrochemical cell. From the results, we determined the equilibrium constant for the dissociation reaction of phenol (K-PhOH). The dissociation constants of phenol varied from 1.0 x 10(-12) mol/kg at 673.4 K and 30.2 MPa to 7.9 x 10(-10) mol/kg at 473.1 K and 29.9 MPa. The semi-empirical model for correlation of the equilibrium constant in subcritical and supercritical water was applied to correlate the data and to discuss the temperature, pressure and density effects on the phenol dissociation constant.