Poly (ethylene terephthalate) (PET) was irradiated in air by gamma rays at dose rate of 28 kGy/h for doses up to 3500 kGy. The influences of gamma irradiation on the glass transition and degree of crystallinity were investigated by differential scanning calorimetry. For amorphous PET, the observed glass transition temperature T-g and step of heat capacity Delta C-p decreased with absorbed dose. These decreases were attributed to a predominant chain scission process. The glass transition temperature may be useful as an indicator of radiation to the PET. The results also showed that such chain scission increases in mobility lead to a greater crystallizability.