1. Four thioethers of 1,4-hydroquinone with glutathione (GSH) were prepared from 1,4-benzoquinone and characterized by 1H-n.m.r. and partly by 13C-n.m.r. spectroscopy. The structures of three additional thioethers were tentatively assigned by u.v. spectroscopy. 2. The corresponding thioethers of 1,4-benzoquinone with GSH were obtained by oxidation of the corresponding 1,4-hydroquinone thioadducts with PbO2 or potassium ferricyanide. 3. Relative redox potentials of the hydroquinone/benzoquinone thioethers were estimated by determination of their redox equilibria with benzoquinone/hydroquinone. The redox potential of the mono-substituted derivative was 30 mV lower, and that of the di-substituted derivatives 70 mV lower, than that of the unsubstituted couple, thus explaining the readiness of sequential oxidation and addition reactions of the produced thioethers. 4. By use of 1,4-[U-14C]benzoquinone the reaction products with GSH were quantified to elucidate the product orientation. As observed with 1,4-benzoquinoneimine and its thioethers, formation of GSSG was not detected at physiological pH. 5. The high susceptibility of particular thioethers of 1,4-hydroquinone towards (aut)oxidation characterizes these products as reactive intermediates rather than as definitive detoxication products.