The activation energies for crystallization in poly(3-hydroxybutyrate) and for nematic-isotropic transition in liquid-crystalline polyesters have been calculated based on data obtained with a dielectric method and differential scanning calorimetry (DSC), respectively. This study explores the feasibility of using a modified Kissinger method to calculate the activation energies of these transitions. It has been found that the activation energy for crystallization in poly(3-hydroxybutyrate) is 109 kJ/mol. The activation energy for nematic-isotropic transition in the liquid crystals is 487 kJ/mol.