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By sequential use of the isothermal charging, the isothermal discharging, the final thermally stimulated discharge current and the final isothermal discharging current techniques, the charge dynamics in highly insulating materials may be investigated. The method is demonstrated for polyethylene terephthalate. The injected charge for a field of 20 MV m-1 and polarization temperatures up to 110°C is almost totally trapped in the material and is released during the heating of the sample at 180°C for a sufficiently long time. A significant current at high temperatures, about 90°C above the poling temperature, was observed proving that it originates from charge detrapping. The final thermally stimulated discharge current peaks shift to higher temperature when the polarization temperature increases, and are characterized by activation energies in the range from 1.03 to 1.56 eV. They allowed the identification of the glass transition around 114°C. The relaxation time of the trapped charge, at 180°C, was determined to be about 3780 s, explaining the very good stability of trapped charge.