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 chi(c)=0.12, crystallized at low temperatures (105 degrees C for 13 min), submitted to physical ageing in a temperature range between 50 and 65 degrees C for different periods of time, sh owed two endothermic peaks. The first peak (P-1) of higher intensity, appeared at a temperature close to the glass transition temperature, T-g, of the amorphous PET, and the other peak (P-2) 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 P-1 and P-2 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 chi(c)=0.32, crystallized at 114 degrees C 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.