The thermal behaviour of nifedipine was studied with the view to understand the various phase transitions between its polymorphs. The focus was on polymorph identification, accompanying morphological changes during crystallization and the nature of the phase transformations. These features were compared to the complexity of the crystallization mechanisms, studied by dynamic differential scanning calorimetry (DSC) heating techniques. DSC, thermogravimetry (TG) established the temperature limits for preparation of amorphous nifedipine from the melt. DSC studies identified that metastable form 13, melting point similar to 163 degrees C, was enantiotropically related to a third modification, form C, which existed at lower temperatures. Form C converted endothermically to form B at similar to 56 degrees C on heating and was shown by hot stage microscopy (HSM) to be accompanied by morphological changes. Modulated temperature differential scanning calorimetry (MTDSC) showed discontinuities in the reversing heat flow signal during crystallization of amorphous nifedipine (from similar to 92 degrees C) to form B, which suggested that a number of polymorphs may nucleate from the melt prior to form B formation. Identification of the number of nifedipine polymorphs included the use of combined DSC-powder X-ray diffraction (PXRD) and variable temperature powder X-ray diffraction (VTPXRD). The crystallization kinetics studied by dynamic DSC heating techniques followed by analysis using the Friedman isoconversion method where values of activation energy (E) and frequency factor (A) were estimated as a function of alpha or extent of conversion (alpha). The variations in E with alpha, from 0.05 to 0.9, for the amorphous to form B conversion could indicate the formation of intermediate polymorphs prior to form B. The form B to form A conversion showed a constancy in E on kinetic analysis from alpha 0.05 to 0.9, which suggested that a constant crystallization mechanism operated during formation of the thermodynamically stable form A. (c) 2006 Elsevier B.V. All rights reserved.