The kinetics of dibenzylether (DBE) hydrothermolysis in hot compressed water has been investigated in a continuous tubular reactor system at a pressure from 225 to 362 bar, temperature from 325 to 390degreesC, and a constant water load of Theta(W) = 100 mol(water)/mol(DBE). Thermal scission of DBE yielded equimolar amounts of benzaldehyde and toluene; the rate constant in supercritical water decreased with water density showing that the volume of activation was positive, as expected for an unimolecular scission reaction. Benzyl alcohol was produced by DBE reversible hydrolysis catalyzed by free, protons from water dissociation. Density and dielectric constant of water decreased the rates of hydrolysis and DBE formation, which means that the transition state complex has a positive volume of activation, and a lower dipolar moment than the reactants. As the dielectric constant raises the reaction rates are lowered, but the equilibrium shifts toward the formation of benzyl alcohol, which is more polar than DBE. The low dipole moment of the transition state complex was further supported by the negative effect of the ionic strength on the rate constant for DBE hydrolysis. (C) 2005 American Institute of Chemical Engineers.