The mechanism of glutathione (GSH) oxidation by a nonheme ferryl species has been investigated. The reaction of [Fe(IV)(O)(N4Py)](2+) (1) with GSH in an aqueous solution leads to the rapid formation of a green intermediate, characterized as the low-spin ferric complex [Fe(III)(SG)(N4Py)](2+) (2) by UV-vis and electron paramagnetic resonance spectroscopies and by high-resolution time-of-flight mass spectrometry. Intermediate 2 decays to form the final products [Fe(II)(OH(2))(N4Py)](2+) and the disulfide GSSG over time. The overall reaction was fit to a three-step process involving rapid quenching of the ferryl by GSH, followed by the formation and decay of 2, which are both second-order processes.