Thiophene endoperoxide 2, which was prepared by photooxygenation of thiophene 1, transfers a sulfur atom (up to 92%) to strained cycloalkenes to form thiiranes when thermolyzed in their presence. The diastereomeric pair cis/trans-cyclooctene (5b) reacted stereoselectively, which speaks for a concerted process rather than open dipolar and/or diradical intermediates. The set of chiral cyclooctenols 5c-e was also investigated, and the relative configurations of the respective thiiranes were assigned by chemical correlation and NMR spectral and X-ray analysis. The first-order kinetics of the process clearly shows that the endoperoxide 2 itself is not the sulfur-transferring species, but it is thermally transformed to the intermediates I and II. Whereas intermediate II is responsible for the competitive formation of elemental sulfur, intermediate I, presumably an oxathiirane, is the active sulfur-transferring species. The episulfidation was compared with the epoxidation by the related dimethyldioxirane, and both show the same qualitative trends in the diastereoselectivity and the reactivity toward the alkenes 5a-e.