Samarium monosulfide in a state amorphous to X-rays was prepared by the successive interaction of lithium naphthalenide with samarium diiodide and a solution of sulfur in THF in inert atmosphere or under reduced pressure under standard (normal) conditions. Samarium monosulfide was transferred to the polycrystalline state (a=5.95 A, NaCl-like structure) by annealing in a vacuum (<0.1 Pa) or in an inert atmosphere at 500–600 °C.