The skeletons of both independent mol-ecules of the carboxylic acid hemihydrate, C(10)H(8)N(2)O(10)·0.5H(2)O, are approximately planar [maximum deviations 0.642 (3) and 0.468 (1) Å]. The deviations arise from the twisting of the nitro groups with respect to the aromatic rings [dihedral angles = 3.24 (2) and 27.01 (1), and 7.87 (1) and 16.37 (2)° in the two molecules]. The crystal structure features inter-molecular O-H⋯O hydrogen bonds, which the link the dicarboxylic acid and water mol-ecules into a supra-molecular layer network.