Oxidation of 2‐benzenesulfonylaminoisothiazolium salts 1, 2 and their imines 3, 4 with hydrogen peroxide gave 1,2,3‐thiadiazine 1‐oxides 5, 6, which were converted into the corresponding 1,2,3‐thiadiazine 1,1‐dioxides 7, 8 using m‐chloroperoxybenzoic acid. Oxidation of 5, 6 with hydrogen peroxide furnished isothiazol‐3(2H)‐one 1,1‐dioxides 9, 10 as ring contraction products.