Molecular weight is an important factor in the processing of polymer materials, and it should be well controlled to obtain desired physical properties in final products for end-use applications. Degradation processes of all kinds, including hydrolytic, thermal, and oxidative degradations, cause chain scission in macromolecules and a reduction in molecular weight. The main purpose of this research is to illustrate the importance of degradation in the drying of poly(ethylene terephthalate) (PET) before processing and the loss of weight and mechanical properties in textile materials during washing. Several techniques were used to investigate the hydrolytic degradation of PET and its effect on changes in molecular weight. Hydrolytic conditions were used to expose fiber-grade PET chips in water at 85 degrees C for different periods of time. Solution viscometry and end-group analysis were used as the main methods for determining the extent of degradation. The experimental results show that PET is susceptible to hydrolysis. Also, we that as the time of retention in hydrolytic condition increased, the molecular weight decreases, but the rate of chain cleavage decreased to some extent, at which point there was no more sensible degradation. The obtained moisture content data confirmed the end-group analysis and viscometry results. Predictive analytical relationships for the estimation of the extent of degradation based on solution viscosity and end-group analysis are presented. (c) 2006 Wiley Periodicals, Inc.