Simultaneous measurements of the dielectric loss tangent (tan&dgr;) and thermoluminescence (TL) or of thermally stimulated current (TSC) and thermoluminescence have been carried out in order to study electron trapping phenomena in polyethylene‐terephthalate (PET). Close correlations between the various measurements have been observed. In tan&dgr; measurements made at 1 kHz and at temperatures ranging from −190° to 150 °C four peaks are observed, viz., at −140° ( &bgr;3), −110° ( &bgr;2), −40° ( &bgr;1), and 150 °C (&agr;). Correlations between the simultaneous measurements of TL and tan&dgr; and that of TL and TSC indicate that the electrons are mainly liberated from their traps by the &bgr;1relaxation. Comparison with available dielectric loss data of oligoethylene‐terephthalate suggests that the &bgr;1relaxation of PET should be attributed to the motions of the carboxyl groups in the main chains. Photoelectroluminescence (PEL) for PET has been observed and degradation of the sample has been noted at the luminous region of the PEL. The coincidence of the PEL spectrum with that of TL suggests that the luminescence center should be attributed to that part of the polymeric chains which are liable to chain scission.