The exothermic peak that is frequently observed during the heating scan of a differential scanning calorimetry (DSC) experiment of poly(ethylene terephthalate) (PET) is due to a cold crystallization process, originating from the rearrangement of amorphous regions into a crystalline phase. In this work the isothermal cold crystallization kinetics of PET was investigated by using DSC, X-ray diffraction and tensile experiments. The isothermal crystallization rate was determined as a function of temperature, and the Avrami analysis was conducted. The results showed that at low temperatures the cold crystallization is a two-regime process, whereas at high temperatures just one stage is observed. The rate constant for isothermal crystallization K increased and the half time of crystallization (t(1/2)) decreased with increasing crystallization temperature. The Avrami exponent n was close to 2, and this corresponds to a disc-like morphology formed by heterogeneous nucleation. Cold crystallization increased the crystallinity and therefore the tensile properties of the samples were enhanced. (c) 2005 Springer Science + Business Media, Inc.