Investigations of mesomorphic and thermo-optical properties of lyotropic nematic-calamitic (N-c) and nematic-discotic (N-d) mesophases are described. The peculiarities of microscopic textures and the temperature and concentration changes of the refractive index and birefringence have been studied. The nematic-isotropic liquid phase transition temperatures and the temperature widths of heterophase regions of the phase transitions from the N-c and N-d mesophases to the isotropic phase have been determined with a high accuracy. Thermic hysteresis in the width of heterophase regions of the phase transitions between nematic mesophases and isotropic liquid have been found.