The physical ageing of semicrystalline poly(ethylene terephthalate) (c-PET) of different crystallinities and morphological structures was studied using differential scanning calorimetry. Samples of c-PET of crystallinity content Xc=0.12, crystallized at low temperatures (105~ for 13 min), submitted to physical ageing in a temperature range between 50 and 65 ~ for different periods of time, showed two endothermic peaks. The first peak (P~) of higher intensity, appeared at a temperature close to the glass transition temperature, Tg, of the amorphous PET, and the other peak (P2) of lower intensity, merged as a shoulder of the first one, at a higher temperature. These peaks have been attributed to the enthalpy relaxation process of two different amorphous regions: one amorphous phase outside the spherulitic structure (interspherulitic amorphous region) and another amorphous phase inside the spherulites (interlamellar amorphous region). The separation between P1 and P2 indicates that DSC, via enthalpy relaxation, is a good technique to detect the real double glass transition of the semicrystalline PET. However, the physical ageing of a semicrystalline PET of • crystallized at 114~ during 1 h, showed a main endothermic peak shifted to a higher temperature, which probably corresponds to the enthalpy relaxation of the more restricted interlamellar amorphous region, and a small endothermic peak at lower temperature which could be a reflection of the hindered interspherulitic amorphous region.