This work is devoted to metallomesogenic lanthanide complexes, which have paramagnetic features and effective luminescence, and which can be used as a basis for developing of magneto- and electro - optical devices.  The magnitude and sign of the dielectric anisotropy both play a leading role in this case. However, any direct measurements of the dielectric anisotropy for the most part of metallomesogenic complexes are highly problematic since they only have a smectic phase, and therefore there is no way to produce macroscopic homogeneous samples. This problem can be solved as shown in our investigation due to similarity between the mechanism of the dielectric polarization in liquid crystal phase and that of electrical birefringence in the isotropic phase. We can estimate the dielectric anisotropy in the liquid  crystal phase by studying the electrical birefringence process in the complexes melt at the temperature above the critical point for the phase transition from the isotropic phase to smectic one. We have studied complexes consisting of ions of rare-earth metals such as  Dy, Er, Gd, Tb, and of alkylsulfate counterions and Schiff bases as being ligands.  The number of methylene groups in aliphatic chains of counterions was varied between 0 and 12. It was found that in the series of the substances under investigation the magnitude as well as the sign of electro- optical effect in the isotropic phase depended on length of the counterions, but not depended on the metal ions used. Thus, dielectric anisotropy of smectic lanthanide complexes can be widely changed by variations of chemical structure of the counterions, namely: the dielectric anisotropy can be positive, negative, or close to zero.