4-Carb-oxy-2-methyl-1H-imidazol-3-ium-5-carboxyl-ate monohydrate.

In the title compound, C(6)H(6)N(2)O(4)·H(2)O, one carboxyl group is deprotonated and one imidazole N atom is protonated. The organic mol-ecule, excluding methyl H atoms, is essentially planar, with an r.m.s. deviation of 0.0156 (1) Å. In the crystal structure, inter-molecular N-H⋯O hydrogen bonds link mol-ecules into chains along the b axis; these chains are further linked via O-H⋯O hydrogen bonds involving the water O atoms and carboxyl O atoms, generating a two-dimensional supra-molecular framework.