The sorption and diffusion of water vapor in poly(ethylene terephthalate) (PET) film were measured by applying a thermogravimetric analyzer (TG-DTA), which customarily has been used to detect the weight loss of a sample with the increase of temperature under a given atmosphere. In this case, we detected the weight gain of PET film by sorption of water vapor under a given humidity at a constant temperature. Sorption-rate curves were successfully obtained in spite of the low solubility of PET film and the presence of Fickian-type curves. The solubility was better described according to the dual-mode sorption model. The diffusion coefficients were determined in their initial slopes by the short-time method. We found that the diffusion coefficient depended on vapor pressure. (C) 2000 John Wiley & Sons, Inc.