Temperature dependences of the refractive indices, birefringence and molecular polarizability in large temperature interval and especially in the region of the direct nematic mesophase - isotropic liquid (for the heating process) and the reverse isotropic liquid - nematic mesophase (for the cooling process) phase transitions have been studied. Thermotropic monomorphic liquid crystalline material with enantiotropic nematic mesophase has been used. Temperature behavior of the order parameter has been determined. Peculiarities of the biphasic regions of the direct crystal - nematic mesophase and nematic mesophase - isotropic liquid and reverse isotropic liquid - nematic mesophase and nematic mesophase - crystal phase transitions have been investigated.