ammonium sulfate, chemical compound, (NH₄)₂SO₄, a colorless-to-gray, rhombohedral crystalline substance that occurs in nature as the mineral mascagnite. It is soluble in water and insoluble in alcohol or liquid ammonia. It is prepared commercially by passing ammonia, obtained from destructive distillation of coal, into sulfuric acid and is used as a fertilizer, in preparing other ammonium compounds, and for fireproofing.