Structural changes in commercial polyethylene terephthalate subjected to gamma-irradiation were investigated by means of differential scanning calorimetry, viscosity measurements and positron annihilation lifetime spectroscopy. Irradiated samples show a lower molecular weight and increased crystallinity, which are attributed to chain scissions of the macromolecules. Positron results support such an interpretation and supply an estimation of the average sizes of the free volume holes and of the defects in the crystalline regions of the polymer.