Isotropic melts of nine metallomesogenic complexes with a smectic-А phase have been studied by the electric birefringence method. Complexes include Dy, Tb, Gd, and Er rare-earth ions, Schiff bases as ligands, and alkyl sulfate or nitric-acid counterions. It has been shown for the first time that both the magnitude and the sign of the electro-optical effect in the isotropic phase for the studied substances are determined by the length of counterions. This means that the dielectric anisotropy of smectic lanthanide complexes can vary widely depending on counterions: it can be positive, negative, or close to zero.