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Previously we reported 1, that solvolytic rearrangement of salts and bases of 2-amino-5-X-5,6-dihydro-4-H-1,3-thiazines \I a,b, where a X=C1, b X=Br\ proceeds with a contraction of cycle and results in salts and bases of 2-amino-5-X-methyl-2-thiazolines \II a-c, where a X=C1, b X=Br, c X=OH\. The reversibility of this reaction is shown now. It was found by means of radiochromatography that hydrolysis of 35S-labelled IIb bromide gives rise to rearranged Ib besides hydrolytic product lie. It was shown, also, that the solvolysis of salts IIa,b in 50% aqueous ethanol, when excess of Na36Cl is present, gives both ordinary substitution products and certain amount of rearranged Ia. In the course of 35-labelled IIb bromide solvolysis in the same conditions the return of bromide ions takes place. It was found that the addition of salts affects the composition of Ib bromide hydrolysis products. This testifies that the composition of reaction products depends on the episulphonium III form present in the med...