A process for producing an ammonium polythiomolybdate of the formula (NH₄)₂Mo₃S₁₃.n H₂O where n is 0, 1 or 2 in which an aqueous ammoniacal molybdate solution is reacted with hydrogen sulfide gas in a closed system at superatmospheric pressure until the H₂S is no longer absorbed by the solution to form a slurry of solid ammonium tetrathiomolybdate containing a portion of the starting molybdenum in a mother liquor containing the balance of the molybdenum and thereafter heat soaking the reaction product in the presence of elemental sulfur to form the desired product.