The advanced isoconverstional method can be used to determine effective activation energies of the nonisothermal crystallization of polymer melts. The application of this method to differential scanning calorimetric data on the crystallization of poly(ethylene terephthalate) yields an activation energy that increases with the extent of crystallization from -270 to 20 kJ . mol(-1). The variation is interpreted in terms of accepted crystallization models.