Measurements are reported for the first time on the D.C. Kerr response and the pre-transitional behaviour of two liquid crystalline isothiocyanates 4-(4′-n-pentyl phenyl)-phenyl isothiocyanate (5NCS) and 4-(4′-n-heptyl phenyl)-phenyl isothiocyanate (7NCS) at temperatures above the smectic-isotropic transition temperatures. The Kerr law has been verified and the real part of the third order nonlinear susceptibilities (χ(3)) above the smectic-isotropic transition are determined for the two liquid crystalline isothiocyanates from the electro-optic Kerr effect (EOKE) experiments. The variation of susceptibility with temperature has also been studied. The results have been compared with the cyanobiphenyls which have been widely studied and are known for their giant optical non-linearities.