New experimental data on the refractive indices and molar refractivities of molten cerium, neodymium, samarium, and gadolinium trichlorides and CeCl3-ACl, SmCl3-AC1, Gd3Cl-ACl (A = Na, 0.5Na-0.5K, K, and Cs) mixtures depending on the temperature and composition are reported. It is shown that the relative deviations of the molar refractivity of mixtures from their magnitudes inherent to the hypothetic ideal solutions increase as the radii of alkali metal cations rises. The difference in variation of the Ln(3+)-ions electron polarizability with the atomic number or ionic radius is revealed for cerium and yttrium subgroups of the lanthanide row.