The main purpose of this research work was to illustrate the extent of hydrolytic degradation of PET caused by warm water and also to separate the influence of moisture, temperature, and orientation in the degradation process. To achieve this purpose, thermal and hydrolytic degradation of PET (Partly Oriented Yarn (POY), Fully Drawn Yarn (FDY) and granule) at temperatures above and below glass transition temperature (T-g) were carried out using a water bath and an electrical oven. Technical methods such as determination of the moisture content, viscometric analysis, carboxylic end group titration, and X-ray diffraction were used to analyze the degradation of PET. The results obtained from different analysis show that the major portion of degradation is carried out by both moisture and heat mutually rather than their entirely individual that have a minor effect. Degradation at lower temperatures from T-g were less prominent and was increased noticeably above T-g. Crystallinity plays a significant role in preventing hydrolytic degradation as the extent of degradation was increased from FDY to granule to POY. X-ray diffraction analysis showed that crystallinity was increased from POY to granule to FDY. Determination of intrinsic viscosity, molecular weight based on viscosity (M-v), chain scissions, and carboxylic end group analysis were the main quantities for the degradation measurement. (C) 2007 Wiley Periodicals, Inc.