AbstractIn order to study the stereochemistry of thiirane oxide decomposition,syn‐ andanti‐2,3‐dideuterothiirane‐1‐oxides andtrans‐2,3‐dideutero‐thiirane‐1‐oxide have been synthesized in three steps fromcis‐ andtrans‐dideuteroethylene via stereospecific formation of the corresponding oxiranes and thiiranes. The presence of the deuterium label was used so as not to perturb the course of the decomposition from that of the parent thiirane oxide. Pyrolysis of the labeled thiirane oxides in both gas and solution phase proceeded with greater than 90% retention of stereochemistry. Analyses were performed by comparison of the infrared spectra of the product mixtures with standard calibrated samples. Attempts to trap possible biradical intermediates with di(p‐anisyl)thioketone gave a low yield of products, possibly derived from addition to thiirane oxide. These results suggest a possible partial contribution of a biradical mechanism to the thiirane oxide decomposition. If bond rotation in the biradical is slow, this mechanism could be the sole mode of