In the present work a comparative study of physical ageing of poly(ethylene terephthalate) (PET) by thermally stimulated polarization currents (TSPC), thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC), is presented. In all cases, physical ageing was carried out by annealing the sample at 60 degrees C for several ageing times (from t(a) = 0 to 264 h). Structural relaxation has been studied by DSC through the analysis of the endothermic peak superposed on the glass transition, which increases and shifts towards higher temperatures as the annealing time increases. Measured current spectra of annealed samples show the appearance of a relaxation peak which we named alpha(a). The evolution of this peak with the annealing time is presented, and it has been related directly to the endothermic peak obtained by DSC, and therefore directly associated with the physical ageing of PET. Experiments carried out show the dipolar origin of the alpha(a) peak, related to the polarization of chainswithin the material when mobility is recovered above T-g.