The addition of hexafluoridophosphate salts (ammonium, silver, thallium or potassium) is usually used to precipitate complex cations from aqueous solutions. It has long been known that PF6- is sensitive towards hydrolysis under acidic conditions Gebala & Jones (1969). J. Inorg. Nucl. Chem. 31, 771-776; Plakhotnyk et al. (2005). J. Fluorine Chem. 126, 27-31]. During the course of our investigation into coinage metal complexes of diphosphine ligands, we used ammonium hexafluoridophosphate in order to crystallize Ag(diphosphine)(2)]PF6 complexes. From these solutions we always obtained needle-like crystals which turned out to be the title compound, 2NH(4)(+)center dot HPO42-. It was received as the hydrolysis product of NH4PF6. The crystals are a new modification of diammonium hydrogen phosphate. In contrast to the previously published polymorph Khan et al. (1972). Acta Cryst. B28, 2065 -2069], Z' of the title compound is 2. In the new modification of the title compound, there are eight molecules of (NH4)(2)(HPO4) in the unit cell. The structure consists of PO3OH and NH4 tetrahedra, held together by O-H center dot center dot center dot O and N-H center dot center dot center dot O hydrogen bonds.