Decomposition reactions of polyvinyl alcohol in high temperature and high pressure water were analyzed in the temperature range of 200 to 400degreesC and pressure range of 1.5 to 33.8 MPa. Qualitative analysis of reaction products showed that not only organic acids but also aromatic Compounds such as benzaldehyde, benzophenone and acetophenone were present in liquid-phase products. These compounds were produced from polyvinyl alcohol by cyclization reactions accompanying thermal degradation and elimination of side chains.