Optical properties of two mesogenic mixtures, code names ZLI 1221and ZLI 1291, possessing same constituents but exhibiting distinct nematic behavior, have been studied. Since both the mixtures possess identical moieties: phenyl cyclohexane, biphenyl cyclohexane and cyclohexyl benzoate, but in different proportions, and exhibit different mesogenic ranges, comparative study of their properties is of great interest. In this work we present a study of the thermal variation of their optical behavior. The variation of polarizability in terms of molecular weight has been obtained for both the mixtures and the thermal dependence of their orientational order parameters have been determined and compared.