The hydrolysis of PET was studied over a wide range of temperatures from 25 to 105°C for duration up to 5 years in water. It was demonstrated that independent of the aging temperature, a decrease in the molar mass was witnessed due to chain scissions that occur during hydrolysis of the ester group. Above Tg, it was shown that chain scissions induced an increase in degree of crystallinity and amorphous layer thickness. However, this phenomenon, chemi-crystallization, was not observed at low temperatures, i.e. below Tg. This change reflects a different behaviour when the polymer is exposed to water in the glassy or rubbery state. This was highlighted by the quite different activation energies below and above Tg, respectively equal to 31 ± 12 and 134 ± 4 kJ/mol.