Degradation of polyvinylchloride (PVC) in high-temperature and supercritical water was studied with a hydrothermal diamond anvil cell to determine phase change characteristics of the reacting polymer with respect to water density. During the reaction period of 500 s, at temperatures between 400 and 500 degrees C and at water den sities from 0 to 930 kg/m(3), PVC particles exhibited clearly defined spreading on the anvil surface that was defined as spread time, S,. The spread times decreased with increasing temperature and the values increased with increasing water density. Analysis of the residues with infrared spectroscopy showed the presence of both polyenes and polyols, whose formation was correlated with water density. From the analyses, nucleophilic substitution of PVC in water was found to be promoted at low temperatures (similar to 400 degrees C) and high water densities (> 830 kg/m(3)), whereas the ionic chain dechlorination was promoted at high temperatures (> similar to 450 degrees C) and low water densities (similar to 750 kg/ m(3)). A reaction pathway is proposed that shows -OH nucleophilic substitution in competition with ionic chain reaction for dechlorination both of which vary with water density. (C) 2007 Wiley Periodicals, Inc.