Glutathione (GSH) and the diastereomeric dibromides derived from syn-(ethyl,methyl)bimane (4,6-diethyl-3,7-dimethyl-1,5-diazabicyclo[3.3.0]octa-3,6-diene-2,8-dione) react to form a small amount of the (GS)2 derivative and substantial amounts of isomeric (cis- and trans-) thia-bridged bimanes [μ-(S)-Mmethyl-methylene,methyl)bimane]. The latter are accompanied by a new compound, glutathione sulfide (GSG). The process is equivalent to the “extraction” of sulfur from two thiol molecules. Formation of GSG occurs via the following steps: (1) displacement of bromide ion by the thiolate anion, GS” from the dibromide to yield a monoalkylthio-monobromo derivative; (2) intramolecular displacement of the remaining bromine by the neighboring alkylthio group to produce a reactive sulfonium ion; (3) disappearance of the intermediate sulfonium ion by either (a) elimination or (b) nucleophilic displacement. (3a) Base causes 1,2-elimination of H+ and the thia-bridged bimane from the intermediate sulfonium ion, yielding “dehydro-GSH” [γ-glu-dehydroala-gly] to which GSH then adds to give two diastereomers of GSG. (3b) Nucleophilic displacement by GS− at the cysteinyl methylene of the intermediate sulfonium ion yields GSG and the thia-bridged bimane. The dibromide diastereomers are produced by bromination of Syn-(ethyl,methyl)bimane in roughly equal amounts, are separable by HPLC, and are stable in nonpolar solvents but interconvert readily in polar solvents. The structure of the trans isomer of μ-(S)-(methylmethylene,methy!)bimane has been established by X-ray crystallography. The present approach to sulfide formation from two thiol molecules in aqueous solution is the mildest yet discovered. GSG does not inhibit the reduction of GSSG by NADPH catalyzed by GSSG reductase.