This paper gives a systematic investigation of Electro-optic Kerr relaxation (τKerr and BDC) studies in the pretransitional region of two homologous series of nematic liquid crystals (i.e.) 4-n-alkyl-4′-cyano biphenyl (nCB) and 4-n-alkoxy-4′-cyano biphenyl (nOCB) series (n = 5 to 8). From the measurement of τKerr we have determined the phenomenological constant ('a') that occurs in the Landau-de Gennes theory on phase transitions. Consequently the Pair Correlation functions (g2) near the pretransitional region and other important physical parameters like the order parameter (SNI) at the nematic-isotropic interface (TNI), the heat of transition 'L' at TNI and the dielectric relaxation time (τD) have also been obtained using different relations. The variations of these parameters within the series have been discussed on the basis of an interesting phenomenon called 'Odd-Even' effect. The details of the results and discussions are presented.