The diphenyl derivative of 4H-1,2,4,5-thiatriazine (5) was prepared by oxidative cyclization of 9. The molecular structure of 5, obtained by X-ray diffraction [orthorhombic, Pna21, a = 9.7746(13) Å, b = 21.692(2) Å, c = 5.6580(8) Å, compares favorably with that predicted with ab initio calculations. The thiatriazine 5 was used as a precursor to the 3,6-diphenyl- 1,2,4,5-thiatriazinyl radical (4) through either oxidation with PbO2, or conversion to and reduction of sulfiminyl chloride 6 with Ph3Sb. The weak ESR quintet (a(N) = 1.03 mT, g = 2.0103) observed in the latter case correlates well with the molecular structure of 4, but the results of DFT calculations are ambiguous. Ab initio calculations show that 4H-1,2,4,5- thiatriazine (I-4H) is the most stable tautomer and is the second most stable isomer among the six possible thiatriazines. All isomeric thiatriazinyl radicals exhibit similar spin distribution patterns. 1,2,4,5-Thiatriazinyl radical (I-R) is calculated to be 23.1 kcal/mol less stable than the most stable 1,2,4,6 isomer II-R.