In the title mol-ecule, C(12)H(12)N(2)O(4)S, the S atom of the thia-diazine ring deviates by 0.5104 (4) Å from the mean plane of the other five atoms [largest deviation = 0.0623 (15) Å] giving a slightly distorted sofa conformation. The carb-oxy H atom was refined as disordered over two sets of sites with refined occupancies of 0.58 (2) and 0.48 (2). This corresponds to rotational disorder of the C=O and O-H groups about the attached C-C bond. In the crystal, O-H⋯O and N-H⋯O hydrogen bonds connect the mol-ecules into chains along [110].