Solid solutions of Sm₂S₃ and Tm₂S ₃ based on BaSm₂S₄ (the structural type of CaFe₂O₄) are obtained. The samples are studied using microprobe analysis and the XRD technique. The length of the solid solution region is determined. The type of ion conductivity and thermodynamics of dissolution of the binary sulfides of Sm₂S₃ and Tm ₂S₃ in BaSm₂S₄ are studied. Partial molar enthalpies of dissolution of binary sulfides in basic thiosamarate are determined. A possible mechanism of defect formation is suggested.