Abstract Singlet oxygen reacts with 2, 5-dimethylthiophene (1) exclusively by (4+ 2)- cycloaddition to yield the thiaozonide 2. The structure of this product is inferred from its 1 H- and 13 C NMR spectra. Neat thiaozonide 2 decomposes violently at room temperature. In aprotic non-polar and in protic polar solvents it is slowly transformed into cis-sulfine 3c and cis-2, 5-dione 4c, which rearranges to the trans-isomer 4t. A mechanism for the ...