The effect of radiation dose (10-30 kGy) on the thermal decomposition of poly(ethylene terephthalate) was studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analysis. The TGA and DSC were carried out in a flowing nitrogen atmosphere at heating rates of 5 and 30degreesC/min for TGA and 10degreesC/min for DSC. The degradation process was composed of three overlapping stages. The second stage, at which a rapid degradation occurs, was studied in detail. The process was found to follow a second-order kinetics and was independent of radiation dose or heating rate. The reaction rate constant (k) was found to depend on the heating rate and iradiation dose. The apparent activation energy (Q) and the logarithm of the preexponential rate constant (log A) were found to decrease linearly with the increase in dose at rates of 3.32 kJ mol(-1) kGy(-1) and 0.177 s(-1) kGy(-1) with intercepts of 249 kJ/mol and 12.26 s(-1) for Q and log A of unirradiated fabric, respectively. A. direct relationship was found between the percentage decrease in Q and log A and the percentage decrease in the temperature corresponding to 50% conversion (T-50%) for samples irradiated at different doses. It was found that a decrease in T-50% by 1% resulted in a decrease in Q and log A by 1.855 and 2.1%, respectively. Changes in Q and log A resulting from radiation, mechanical and thermal treatments, or their combinations can be predicted from the shift in T-50%. The history of the fibers substantially affected the thermal properties. DSC and XRD studies revealed changes in the fabric crystallinity. DSC measurements indicated a linear increase in heat of fusion with dose increase at a rate of 0.855 kJ kg(-1) kGy(-1). XRD analysis confirmed structural changes, rearrangement by plane rotations, and formation of compact crystalline lattice with patterns characterizing irradiated samples. An attempt to explain the dependency of the apparent activation energy on dose was given. (C) 2004 Wiley Periodicals, Inc.