The chemical, like the physical, properties of 1,3,5,2,4-trithiadiazepines broadly support their 10π aromatic nature. The parent compound (1) is thermally stable, though photochemically labile, does not undergo cycloaddition reactions characteristic of sulphur di-imides, is inert towards acids, amines, iodomethane, catalytic hydrogenation, and reacts only slowly with m-chloroperbenzoic acid. It undergoes electrophilic aromatic substitution smoothly to give mono- and di-nitro and mono- and di-bromo derivatives, though it could not be acylated. It is, however, sensitive to hydroxide and t-butoxide ions; treatment with potassium t-butoxide and iodomethane gives 3,4-bis(methylthio)1,2,5-thiadiazole (4). Triphenylphosphine similarly causes ring contraction of trithiadiazepine (3) to the 1,2,5-thiadiazole (5). The properties of the benzo compound (2) are similar to those of the monocyclic compound (1), though it is generally less stable and more reactive than (1), in agreement with some bond alternation in the bicyclic structure. Treatment of compound (2) with bromine gives bis(1,3,2-benzodithiazolium) bromide tribromide (11). Mechanisms are proposed (Schemes 1, 2, and 4) for the various trithiadiazepine ring contractions reported.