Physical aging characteristics of poly(ethylene terephthalate) have been evaluated in relationship to volume fraction levels of crystallinity up to 25%. Changes in the enthalpies of relaxation, monitored at aging temperatures from 55 to 65 degrees C, are found to give good fits with the Cowie-Ferguson model. Overall equilibrium enthalpy of relaxation values decrease linearly with increased crystallinity. They increase with decreased aging temperature, providing extrapolated lower temperature results that are validated in terms of specific heat relationships. Activation energies for enthalpic relaxations are found to increase from 337 to 361 kJ/mole as crystallinity increases up to 25%. Overall relaxation endotherms are further resolved into contributions from interspherulitic and intraspherulitic amorphous regions. Interspherulitic, equilibrium enthalpies of relaxation decrease with increased levels of crystallinity, while intraspherulic values show corresponding increases. Characteristic relaxation times of the intraspherulic regions increase greatly, as levels of crystallinity increase; however, interspherulitic relaxation times decrease very slightly. Dynamic differential scanning calorimetry results show two glass transitions in the case of a 25% crystalline sample and a single transition for noncrystallized material. (c) 2007 Wiley Periodicals, Inc.