The reaction of singlet oxygen with variously substituted oxathiins 1 affords dicarbonyl compounds 4 and/or ketosulfoxides 7 and 8 depending on the nature of the substituent at C3 and on the reaction conditions. The normal fragmentation of dioxetanes 2 to 4 competes with an intramolecular oxygen transfer to ring sulfur, which leads to 7 and 8, presumably via the labile epoxides 5. This new pathway is promoted by electron-withdrawing groups at C3 and, for unsubstituted and monosubstituted amide derivatives 1h and 1i, respectively, by the solvent basicity. Chemical experiments support the intermediacy of epoxides 5 for 7, whereas they are not conclusive for 8. However, the formation of the latter compounds appears to be favored by polar solvents and cation-stabilizing groups at C2 as phenyl or methyl, and these observations may be well accounted for by the suggested pathway from 5 through charged species as E. Direct oxidation by singlet oxygen to sulfur is unsignificant, except for the amide series and for 1g or when the oxygenation is carried out in methanol.