Ammonium phosphomolybdate is the inorganic salt of phosphomolybdic acid with the chemical formula (NH<sub>4</sub>)<sub>3</sub>PMo<sub>12</sub>O<sub>40</sub>. The salt contains the phosphomolybdate anion, a well known heteropolymetalate of the Keggin structural class.
Ammonium phosphomolybdate can be made by heating ammonium orthomolybdate combined with phosphoric acid and nitric acid, yielding ammonium nitrate, water, and a yellow precipitate of ammonium phosphomolybdate is obtained.
Normally, it often exists as a hexahydrate, a dark yellow fine crystal which is poorly soluble in water.
It is also obtained as a test result for phosphate ions.