The behavior of polyacrylonitrile synthesized by anionic polymerization (A-PAN) was studied during heating up to 550 degrees C in dynamic conditions in inert atmosphere and in air. Endo- and exothermic effects without mass losses were revealed within the 400-550 degrees C. The data of differential scanning calorimetry (DSC), differential thermal analysis (DTA) and X-ray analysis confirm the formation of mesogenic structures determined by the conformation of initial A-PAN samples. Changes in quantitative characteristics of phase states were explained by reorganization of the formed structures resulting from thermotropic mesophase transition from cyclic macromolecules to nuclei of turbostratic carbon structure. (C) 2014 Published by Elsevier B.V.