The asymmetric unit of the second polymorph of the title compound, C2H9NO6P2, contains one molecule existing as a zwitterion. The N atom of the ammonio group is protonated and one of the phosphonic acid groups is deprotonated. Bond lengths and angles are similar in both polymorphs. Besides the differences in cell parameters, the most significant structural difference between this structure and that of the first polymorph [Dudko, Bon, Kozachkova, Tsarik & Pekhno (2008), Ukr. Khim. Zh. 74, 104–106] is the presence of strong symmetric hydrogen bonds between neighbouring phosphonate groups. H atoms involved in these hydrogen bonds are located at inversion centres and O...O distances are observed in the range 2.458 (5)–2.523 (5) . These bonds and additional O—H...O and N—H...O hydrogen bonds interlink the molecules, giving a three-dimensional supromolecular network.