Optical studies as a function of temperature have been conducted on two liquid crystalline mixtures: Mixture 1: containing phenyl cyclohexane, biphenyl cyclohexane and cyclohexane carboxylate (Code Name: ZLI 1701); Mixture 2: containing phenyl cyclohexane, cyano cyclohexane and cyclohexane carboxylate. (Code Name: ZLI 1800-000). Each of the mixtures exhibits only the nematic phase and has similar temperature ranges for the same. Both the mixtures contain phenyl cyclohexane and cyclohexane carboxylate groups. However, mixture 1 contains biphenyl cyclohexane whereas mixture 2 contains cyano cyclohexane. Untill now no systematic study on the optical properties of the two mixtures have been reported. Our interest is to compare the macroscopic properties of the two mesogenic mixtures and to study the effect of replacing the biphenyl cyclohexane group in mixture 1 by cyano cyclohexane group in mixture 2. With this aim in view the variation of the refractive indices with temperature has been determined for both samples. We have also determined the thermal dependence of the orientational order parameter of the two mixtures from the optical studies.