Reactions of (XSN)n (n = 1, 3; X = F, Cl) with perfluorobutadiene provide the 1,2-thiazines I. The structure of I (X = Cl) was detd. by single-crystal X-ray diffraction anal. From the mol. structure it is evident that a formal [4 + 2]cycloaddn. has occurred. Fluorination with KF converts I (X = Cl) almost quant. into I (X = F). I (X = F) does not exhibit fluorinating properties towards chloroalkanes but undergoes reactions with MeOH and Me3SiNMe2 to form I (X = OMe, NMe2) resp. 1,2-Thiazine oxide II is obtained from I and (Me3Si)2O. Treatment of I (X = Cl) with AgAsF6 in SO2 yields perfluoro-1,2-thiazin-1-ium hexafluoroarsenate as the main product. No reaction is obsd. between FSH and hexachlorobutadiene. 1,2-Thiazines are also prepd. from tetrachlorothiophene dioxide and FSN. The structures are supported by F abstraction reactions with formation of the expected 1,2-thiazin-1-ium ions. These cations are very sensitive to hydrolysis and are converted into the corresponding ketones. The very unstable intermediate CF3SSN, formed on treating Me3SiSCF3 or B(SCF3)3 with FSN, decomps. to CF3SxCF3 (x = 2-4), S4N4, S8, and N2. No addn. product can be detected when CF3SSN prepd. in situ is treated with dienes.