The synthesis and chemical reactions of a series of thiadiaziridine 1, l-dioxides (2) are described. The thermal stability is highly dependent on the R group and in one case (R= tert- octyl) it is postulated that the thermolysis initially produces a diradical intermediate which can be trapped or further fragments to give a nitrene. A temperature-dependent NMR study on the coalescence of the diastereotopic a-methyl protons of 2b gives a AG*= 20 kcal mol-'.