In this paper a study of surface charge decay has been made on polyethylene terephthalate thermoelectrets prepared from foils of thicknesses 6, 25, 75,and 250 &mgr; under various conditions of polarization. Initially, heterocharges have been observed on both the surfaces, while final charges one month after preparation were homo‐ on the cathode surface and hetero‐ on the anode surface. Infrared absorption characteristics have also been studied. Experimental evidence is given for dipolar alignment during polarization in this polymer. Dipole alignment and space charge trapping are shown to be the main causes of electret polarization in polyethylene terephthalate. Relaxations of dipoles and trap hopping processes are responsible for the charge decay.