The syntheses of several N-aryl-3-amino-4-nitroisothiazol-5(2H)-imines 12 from 3,3-diamino-2-nitrothioacrylamides 11 are reported (Scheme 3). In polar solvents, a spontaneous isomerization of some of the prepared isothiazol-5(2H)-imines 12 yielded benzothiazoles 13 (Scheme 4). In the case of 2-alkyl-substituted derivatives of type 12, the isomerization occurred only at higher temperatures. Electronic influences of different substituents on the rate of the isomerization were studied, and a polar reaction mechanism is proposed in Scheme 6. The structures of 12e and 13e were established by X-ray crystallography. Conformational analyses of 3-(methylamino)-2-nitro-N-phenyl-3-(pyrrolidin-1-yl)thioacrylamide (11l) by NMR and X-ray methods were performed with the aim of explaining the distinct behavior of this amide towards Br2 or diethyl azodicarboxylate.