The solid-solution regions in the MeSm₂S₄-MeS and MeSm₂S₄-Sm₂S₃ (Me = Ca, Ba) systems are revealed. The average ion, cation, and anion transport number of the synthesized solid electrolytes xSm₂S₃[Ca(Ba)S] • (100 - x)Ca(Ba)Sm₂S₄ (x = 1-10 mol %) are determined by the electromotive force (emf) method with the use of concentration cells with and without transfer. In the phases under investigation, the ion transfer in the temperature range 673-723 K is provided by sulfide ions (t_S2 = 1.00±0.02). The diffusion coefficients of S^2- ions in the solid electrolytes are determined by potentiostatic chronoamperometry. A vacancy mechanism of defect formation is proposed. It is demonstrated that the transport characteristics of the solid electrolytes based on the CaSm ₂S₄ compound are worse than those of the solid electrolytes based on the BaSm₂S₄ compound.