Relaxation processes at temperatures above 20 °C in semicrystalline polyethylene-terephthalate have been studied using thermally stimulated depolarization currents (TSDC). The discharge curve shows three relaxation peaks (αc, ρc and ρ*) whose positions and intensities depend on the polarization conditions and the crystallinity. Relaxations αc and ρc are heteropolar, while ρ* may be homopolar or heteropolar according to the polarization temperature used. The effect of the crystallinity on these relaxations has been analysed by the thermal steps stimulation (TSS) method applied to an amorphous sample. Results show that αc is fundamentally a dipolar relaxation associated with the amorphous interlamellar zone. The relaxation ρc is associated with the release of a free charge trapped in the amorphous regions, and ρ* is a Maxwell–Wagner–Sillars relaxation associated with crystalline – amorphous interphases. For polarization temperatures above 150 °C, two relaxations are observed only as a consequence of overlapping ρC and ρ* relaxations.