In the anion of the title compound, C(5)H(12)N(+)·C(10)H(8)NO(5)S(-), the thia-zine ring adopts a distorted half-chair conformation and the enolic H atom is involved in an intra-molecular O-H⋯O hydrogen bond, forming a six-membered ring. The anions and cations are connected via N-H⋯N and N-H⋯O inter-actions.