5-Diazoimidazoles and 5-diazopyrazoles have been shown to react with acyl isothiocyanates yielding the imidazo- and pyrazolo5,1-d1,2,3,5thiatriazines stabilized by a nonbonded Sa=O interaction. In contrast to acyl isothiocyanates, alkyl-, aryl-, and arylsulfonyl isothiocyanates do not react with 5-diazoazoles. The nature and the strength of stabilizing intramolecular interaction between non-bonded S and O atoms have been studied by X-ray analysis for mono crystals and DFT calculations for selected azolo5,1-d1,2,3,5thiatriazines. The interaction was described in terms of Weinhold covalence ratio factors, NBO, and AIM schemes. The reaction discovered was used to develop an efficient approach toward the new 8-substituted 4-ethoxycarbonylimino-4-benzoyl- and 4-(3,4,5,6-tetrafluorobenzoyl)iminoimidazo(pyrazolo)5,1-d1,2,3, 5 thiatriazines.