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The crystal studied of the title compound, C7H9N2O4+·ClO4−·C7H8N2O4·H2O, was found to be a racemic twin with a 0.72 (18):0.28 (18) domain ratio. The hydrazidium group is close to planar, with an r.m.s deviation of 0.105 Å; the hydrazide group deviates more from planarity, with an r.m.s deviation of 0.174 Å. In the crystal, the hydrazidium cation, hydrazide molecule, perchlorate anions and water molecules are linked through O—H...O, N—H...O and C—H...O hydrogen bonds into a three-dimensional supramolecular network. In addition, the benzene rings of the hydrazidium and hydrazide units are connected via π–π interactions into infinite chains along the c axis; the centroid–centroid distances are 3.486 (3) and 3.559 (3) Å.