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In the title compound, C7H8N2O4·H2O, the imidazole N atom is protonated and one of the carboxylate groups is deprontonated, forming a zwitterion. The two carboxyl groups are are approximately coplanar with the imidazole ring [O—C—C—C torsion angles = −176.8 (2) and 2.9 (4)° for one group and −4.6 (3) and 176.4 (2)° for the other] and have an intramolecular O—H...O hydrogen bond between them. The water molecule is linked to the organic molecules via an N—H...O hydrogen bonds. Intermolecular O—H...O and N—H...O hydrogen bonds are found in the crystal structure.