The photolysis of trans-4,5-dihydroxy-1,2-dithiacyclohexane in aqueous and CH2Cl2 solution yields two isomers of 2,3-dihydroxy-1-mercaptotetrahydrothiophene, characterized by H-1 and C-13 NMR and negative ion electrospray mass spectrometry. Product formation in water is independent of the presence of oxygen and the concentration of trans-4,5-dihydroxy-1,2-dithia-cyclohexane and involves intramolecular 1,5-H-transfer, followed by cyclization through thiophilic or nucleophilic addition, or 1,2-H-transfer, followed by cyclization through the recombination of a sulfur- and a carbon-centered radical. In contrast, the quantum yields for reaction in CH2Cl2 solutions are dependent on both oxygen concentration and on the concentration of trans-4,5-dihydroxy-1,2-dithiacyclohexane. The latter results are consistent with an intermolecular H-transfer between an initial dithiyl diradical and trans-4,5-dihydroxy-1,2-dithia-cyclohexane. Time-resolved laser flash photolysis studies indicate rapid product formation on the submicrosecond time scale and support the intermediacy of alpha-mercaptoalkyl radicals.