A series of para-substituted aryldithiadiazolylium salts [p-XC,H,cNSSh] [Y] (X = H, Me, CF,, MeO. MeS, NO,, F, CI, Br, CN or CNSSN'; Y = CI or AsF,) and [p-XC,H,CNSNS] [AsF,] (X = H, Me, CF,, MeO, MeS, NO,, F, CI, Br or CNSNS') has been prepared. An electrochemical study of both 1,2,3,5-and 1.3.2.4dithiadiazolylium derivatives, as the hexafluoroarsenate(v) salts, showed reversible redox behaviour, forming the corresponding dithiadiazolyl radicals. The half-wave reduction potentials, €&red), for the two isomeric dithiadiazolylium cations showed linear free-energy relationships between €$(red) and the Hammett value, ( T , ,

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When 4-phenyl-I ,2,3,5-dithiadiazole dimer (PhCNSS&J2 dehalogenates cyclotetrathiatriazenium chloride, [S4N3]CI, and cyclopentathiazenium chloride, [S5N5]CI, in acetonitrile, novel ring contractions occur to give the title compounds (1) and (2).