The structure of the title compound, C(9)H(7)NO(6), is essentially planar [maximum deviation 0.284 (2)Å] except for the methyl H atoms. The crystal structure is stabilized by asymmetric O-H⋯O hydrogen bonds linking the hydrogen carboxyl-ates into pairs around the inversion centres. There is also π-π stacking of the benzene rings [centroid-centroid distance 3.6912 (12) Å].