Metallomesogenic lanthanide complexes can be used as a basis for developing 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 the 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 due to the similarity between the mechanism of the dielectric polarization in the liquid crystal phase and that of the electrical birefringence in the isotropic phase. Thereby, one can estimate the dielectric anisotropy in the liquid crystal phase by studying the electrical birefringence in the isotropic melt. 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.