The title compound, (NH4)4(C4H12N5)2[V10O28]·6H2O, crystallizes with the decavanadate anion placed on an inversion centre in space group P\overline{1}. This anion is surrounded by a first shell of ammonium cations and water molecules, forming efficient N—H...O and O—H...O hydrogen bonds. A second shell includes metforminium monocations with a twisted geometry, also forming numerous intermolecular hydrogen bonds. The complex three-dimensional network of non-covalent interactions affords a crystal structure in which the cations and anions are densely packed.
