Polymerisation and polymer modification are often carried out by means of radiation methods, since irradiation can modify the structure of materials and improve their performance. On the other hand, combined action of ionising radiation and oxygen may induce a degradation of the polymer. Physical changes of different polyesters subjected to gamma irradiation up to 1 MGy are discussed and evidenced by various techniques, mainly viscometry, differential scanning calorimetry and positron annihilation lifetime spectroscopy (PALS). Viscometry monitors variations in the molecular weight. Calorimetric measurements allow one to detect crystallinity changes. PALS evidences possible irradiation effects on the free volume; furthermore, it supplies information on the chemical modifications induced by oxygen. The results can be explained in terms of the main mechanisms induced by gamma irradiation (chain scission, crosslinking and oxydative degradation), although their importance depends on the dose.