The kinetics of isothermal melt-crystallization of poly (butylene terephthalate) (PBT) has been evaluated in a wide temperature range from below the glass transition temperature T-g to close to the equilibrium melting temperature, using fast scanning chip calorimetry. Analysis of heating scans recorded after pre-defined crystallization times permitted gaining valuable information about the effect of crystallization on the glass transition, the cold-crystallization kinetics and stability of crystals formed at the crystallization temperature and during heating. As one of the main conclusions, it is suggested that reorganization on heating involves melting of crystals without formation of a completely relaxed and isotropic melt (at the length scale of a crystal) but a non-isotropic cluster of chain segments which turn into a more stable crystal by a fast click-but not classical secondary nucleation/growth-mechanism. (C) 2017 Elsevier Ltd. All rights reserved.