Singlet oxygen reacts with thietane, 1, to give the sulfoxide, 1SO, and a trace of sulfone, 1SO(2). A mechanism which involves a novel substrate catalyzed interconversion of a persulfoxide and thiadioxirane intermediate is proposed. The data which support the reaction of the persulfoxide with 1 by attack at sulfonium sulfur include the facts that Ph(2)SO and 1 compete for a common intermediate and physical quenching is suppressed by increasing concentrations of 1. The unique ability of 1 to catalyze its own oxidation is a result of a small C-S-C angle which allows an unencumbered approach to the sulfonium sulfur. In contrast, the larger C-S-C angle in Et(2)S sterically precludes the catalysis step and forces direct collapse of the persulfoxide to the thiadioxirane intermediate.