The crystal polymorphism, transformation, and morphologies in chloroform solvent-cast poly (hexamethylene terephthalate) (PHT) were examined by using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and temperature in situ transmission electron microscopy (TEM). Solvent-induced crystallization of PHT at room temperature yielded an initial crystal of gamma-form, as confirmed by WAXD. Upon DSC scanning, the original gamma-form in PHT exhibited three endothermic peaks, whose origins and association were carefully analyzed. The first peak, much smaller than the other two, is in the temperature range of ca. 100-120 degrees C. It was found that the solvent-induced gamma-form was transformed to beta-form at 125 degrees C via a solid-to-solid transformation mechanism. In addition, WAXD showed that gamma- and beta-forms co-existed in the temperature range of 100-125 degrees C. These mixed crystal forms were further identified using TEM, and the selected-area electron diffraction (ED) patterns revealed that both gamma- and beta-form, crystals co-existed and were packed within the same spherulite. Solid-solid transformation from the solvent-induced gamma-form to beta-form in PHT upon heat scanning was presented with evidence and discussed. (c) 2006 Elsevier Ltd. All rights reserved.