Bis(ammonium) tris(hexaaquamagnesium) tetrakis(hydrogen phosphite).

The structure of the title compound, (NH4)2[Mg(H2O)6]3(HPO3)4, consists of [Mg(H2O)6]2+ and (NH4)+ cations and (HPO3)2- anions held together by an intricate network of hydrogen bonds involving all H atoms except for one linked directly to a P atom. The Mg2+ cations are octahedrally coordinated by six water molecules. One of the Mg atoms is located on a site with 2/m symmetry, whereas the other Mg atom and the P and N atoms occupy sites with m symmetry.