Considerable attention has focused on the oxidation of sulfur compounds in connection with the study of phot odynamic action with biological substrates. Irradiation of alkyl sulfides in the presence of a photosensitizer and oxygen gives the corresponding sulfoxides in approximately quantitative yields after consumption of one half mole of oxygen per mole of sulfide. These reactions presumably involve an intermediate such as a peroxy sulfoxide or possibly a cyclic peroxide which is capable of converting another sulfide molecule to the corresponding sulfoxide or rearranging more slowly to the isomeric sulfone. Clean conversions to sulfoxides were observed upon direct photolysis of sulfides in air. In this case an excited charge transfer complex mechanism between oxygen as an electron acceptor and the sulfide as an electron donor has been proposed. A number of biologically important molecules containing disulfide bond are known to be succeptible to photodynamic action. Photosensitized oxygenation of the simplest acyclic disulfides has been reported to give the corresponding thiolsulfinates in good yield. The molecular oxygen reacts with thiones very efficiently to yield the corresponding carbonyl compounds, and singlet oxygen may be involved as the reactive species. Several mechanistic studies of the photooxygenation of vinyl sulfides show oxidative cleavage of the carbon-carbon double bond to take place via the corresponding 1,2-dioxetane intermediate. On the other hand, tri- and tetrasubstituted thioethylenes give products due to oxidative cleavage of the C-S bond. The oxidation of vinyl sulfides possessing an activated double bond and allylic hydrogen atoms affords both 1,2-dioxetane mode products and ene reaction mode products. The dioxetane mode is favored exclusively in protic solvents and slightly favored over the ene mode in aprotic solvents. The photooxidation of vinyl sulfides sensitized by 9,10-dicyanoanthracene has also been studied, and may proceed by electron transfer from the sulfur atom to the sensitizer which again transfers its electron to oxygen to give a superoxide anion radical.