The reaction progress of methylene chloride under hydrothermal conditions was pursued using a corrosion-resistant flow reactor made from titanium tubing, paying attention to minimize the contribution of the preheating zone for the reaction. The relation between the CH2Cl2 conversion and residence time showed that the hydrolysis is first order in CH2Cl2. The rate constant increased monotonically as the temperature was raised but dropped to a large extent above the critical temperature. A relatively small increase of the rate constant was observed in the presence of NaCl under 360 degreesC. The kinetic behavior could be explained based on a S(N)2 mechanism under subcritical conditions, and the sudden drop of the rate constant at the critical point was attributed to the abrupt decrease of the dielectric constant of water. The salt effect wats also analyzed along the same context. The reaction rate and the detailed reaction progress, however, under supercritical conditions are not yet fully understood.