The preparation process is divided into two main groups depending on the phase composition of the polycrystalline reaction product: the formation of Ln2O2S as the only polycrystalline phase and the preparation of several polycrystalline Ln2O2S phases. Based on the established chemistry of the interaction of metallic samarium with sulfur in a sealed ampoule, phase equilibria in the Sm – Sm2S3 – Sm2O3 system, the synthesis parameters of a mixture containing more than 98.5 mol.% Solid solution are determined Sm1+x S1-x ([Sm])1-y [ ]x)2x (Ñ? = 0–0,035, y = 0–1), saturated with excess samarium. According to the results of MSA, the composition of the eutectic was 65 mol% Sm3S4. The composition of the double eutectic has coordinates 0.65 Sm3S4, - 0.35 Sm2O2S and a calculated melting point of 1700K. As a result, the goal of the work was achieved.