Studies of temperature behavior of the mean refractive index, ordinary and extraordinary refractive indices, and birefringence have been carried out for three nematogens. The thermo-morphologic properties of the biphasic regions of the direct nematic-isotropic liquid and reverse isotropic liquid-nematic phase transitions have been investigated. The 4-n-alkyl-4 '-oxycyanobiphenyls (n = 5, 6, 7) were objects of our study. Studies have been carried out for the circular cycle as the heating-cooling. Temperature behavior of the order parameter in nematic mesophase and at the nematic-isotropic liquid and isotropic liquid-nematic phase transition is discussed.