The static Kerr effect and third order non-linearity were investigated for mixtures of the tolane-base liquid crystals within the isotropic phase in the neighborhood of the nematic phase. The nematic mixtures are characterized by components having similar molecular structure, i.e., the mixtures comprised by 4'-pentyl-3-fluoro-4-isothiocyanatotolane (5TOLF) and 4'-heptyl-3fluoro-4-isothiocyanatotolane (7TOLF) which have a positive and large Kerr constant. The nematic to isotropic transition temperatures and nematic to crystalline temperatures for these mixtures were measured using DSC and polarizing microscopy methods. The nematic mixture of M(3) (%50 5TOLF + %50 7TOLF) shows the largest nematic range. The linear dependence of ( T - T*)(-1) on the Kerr constant is found to be in good agreement with the predications of the Landau-De Gennes model. The hypothetical second order phase transition temperatures and susceptibility values were determined for these compounds.