Abstract The electric birefringence in the isotropic melts of two smectic nCB homologues with n = 10, 12 was investigated. Measurements were performed above clearing temperature in a wide temperature range of about 60 degrees with an experimental error of less than 1 %. It is shown that the temperature dependence of the reciprocal of the Kerr constant in the isotropic melt of 10CB and 12CB liquid crystals had the form 1/~(T–T*), where T* is a temperature of virtual phase transition of the second order and it allows to make a description of the electro-optical properties of the isotropic melt by the Landau – de Gennes model with orientation and coordination order parameters. It was found that in the isotropic state a sharp change of electrooptic properties takes place, whereby the dependence 1/~(T*–T) is approximated by two straight lines with different slopes. This fact, as well as availability for each of the studied liquid crystals of two temperature values T* is caused by a structural transition in the isotropic phase. It can be assumed that the transition in the smectic liquid crystals 10CB and 12CB occurs between the low-temperature melt, characterized by short-range orientation and coordination orders and the high temperature melt, which retains only the short-range orientation order.