Abstract: A process for preparation of an ammonium polythiomolybdate or a hydrate thereof of the formula (NH4)2Mo3S13.n H2O where n is 0, 1 or 2, in which aqueous ammonium monosulfide, elemental sulfur and molybdic oxide are reacted in a sealed reactor, with hydrogen sulfide gas being fed into the reactor until the hydrogen sulfide is no longer absorbed by the reaction mixture to form a reaction product and then heating the reaction product in a sealed reactor, the concentration of ammonium monosulfide in the reaction mixture being controlled throughout the reaction such that reactor pressure does not exceed 700 kPa (100 psig).