In this study, the dynamics of the changes of the morphological properties in the nematic mesophase and in the heterophase regions of the phase transitions between the nematic mesophase and the isotropic liquid for the thermotropic mesogens have been investigated. Experimentally, we have obtained the thermal hysteresis for the investigated phase transitions. For the analysis of the phase transition temperatures and the peculiarities of the heterophase regions, the mean field theory has been used. (C) 2002 Elsevier Science Ltd. All rights reserved.