The title compound, C11H10N2O4S, crystallized with two independent molecules (A and B) in the asymmetric unit. They differ primarily in the rotational orientation of the five membered heterocyclic ring. In molecule A this ring is inclined to the benzene ring by 48.17 (8)degrees, while in molecule B the same dihedral angle is 23.07 (8)degrees. In each molecule there is an intramolecular O -H" " "O hydrogen bond involving the adjacent hydroxyl group and the ester carbonyl O atom. In the crystal, the A molecules are linked via pairs of N-H center dot center dot center dot N hydrogen bonds, forming inversion dimers. These dimers are linked to the B molecules via N-H.center dot O, C-H center dot center dot center dot O and C-H center dot center dot center dot=S hydrogen bonds forming corrugated sheets lying parallel to (102).