Diphenylether (DPE) interacts with supercritical water at temperatures of 415-480 degrees C and forms reaction products which change with the water density. At low water density, from 0 to approximately 0.3 g/cc, the conversion of DPE decreases gradually. Products are typical for radical-type polycondensation reactions, i.e. diphenyl, phenyl-DPE, phenoxy-diphenyl, etc. At water densities greater than 0.4 g/cc these products vanish, the conversion of DPE increases again and forms phenol as sole reaction product. This component indicates ionic hydrolysis as the governing chemistry. Validation of the data from high water density measurements support an S(N)1 proton-catalyzed mechanism for the hydrolysis of DPE. Strong effects of water density become apparent on protonation of DPE and an ionic dissociation of phenol, as regards the negative values of the reaction volumes of these equilibria.