The hydrolytic stability of poly(ethylene terephthalate) (PET) has already been largely reported. The chemical reactions induced by damp-heat exposure are well-known, and various kinetic expressions for the degradation have been presented. Using the data from previous studies, a new model for degradation is proposed. This model combines the effect of temperature and humidity in a single equation. Three parameters are utilized: the classical pair of activation energy (Ea) and pre-exponential factor (f(0)), and the reaction order (n) to the relative humidity (RH). The model may be used to fit the degradation data from various sources describing the hydrolysis over a large range of conditions (40-100 % RH, 60-160 degrees C). In addition a prediction of the crystallinity changes brought about by hydrolytic chain scission was performed. Prediction of useful lifetime in moist heat is also possible (hydrolysis of 0.2% ester moieties in the polymer). (C) 2018 Elsevier Ltd. All rights reserved.