The physical ageing of amorphous poly(ethylene terephthalate)(PET) is studied by differential scanning calorimetry (DSC) and by thermally stimulated currents (TSC). The ageing of PET is studied for a range of ageing temperatures (T_a) between 50 and 70 °C at ageing times (t_a) between 15 min and 25 h. The endothermic peak which appears in the glass-transition region is studied by the DSC technique and the maximum value of the enthalpy relaxation can be observed for T_a between 60 and 65 °C. At 70 °C the structural relaxation to equilibrium is accomplished at 4 h. The physical ageing effect on the electret properties is studied by polarizing the PET sample before (P + A samples) or after the ageing (A + P samples) and further analysis by the TSC technique. In both cases, the effect of physical ageing on the position and charge of the α dipolar peak is observed. The charge of the dipolar peak decreases with t_a, the decay in the P + A samples being sharper. The position of the α peak proves to be more sensitive to the physical ageing than the temperature of the DSC endothermic peak. The TSC results are correlated with the calorimetric ones.