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Reduced glutathione (GSH) is a cornerstone of the antioxidant stratagem for eukaryotes and some prokaryotes. Hypochlorous acid (HOCl), which is produced by neutrophilic myeloperoxidase, reacts rapidly with excess GSH to yield mainly oxidized glutathione (GSSG). GSSG can be further oxidized to give first N-chloro derivatives and, later, higher oxidation states at the S centers. Under certain conditions, another major species that is observed during the oxidation of GSH by HOCl (and a minor species for other oxidants) exhibits a molecular mass that is 30 mass units heavier than GSH. This GSH+2O-2H species, which has been employed as a biomarker for oxidative stress, has been previously proposed to be a sulfonamide. Employing NMR spectroscopy and mass spectrometry, we demonstrate that the GSH+2O-2H species is indeed a nine-membered cyclic sulfonamide. Alternative formulations, including six-membered 1,2,5-oxathiazine heterocycles, have been ruled out. Remarkably, the sulfonamide exists as a 2:1 equilibrium mixture of two diastereomers. Isotope tracer studies have demonstrated that it is the Glu C αcenter that has undergone racemization. It is proposed that the racemization takes place via an acyclic imine−sulfinic acid intermediate. The glutathione sulfonamides are stable products of GSH that have been detected in physiological systems. Elucidation of the structures of the glutathione sulfonamides provides further impetus to explore their potential as biomarkers of hypochlorous acid formation. [ABSTRACT FROM AUTHOR]