Thienoanneleted 6aλ4‐Thia‐1,6‐diazapentalenes by Baseinduced Dimerisation of 5‐Methyl‐isothiazolium SaltsIsothiazolium salts2and3are easily available by reaction of (Z/E)‐β‐thiocyanatovinyl aldehydes with primary aliphatic and aromatic amines in acetic acid or with aromatic amine hydrochlorides, respectively. Preparative advantages of this reaction are demonstrated and discussed. Reaction of3with secondary amines results in an unexpected formation of 6aλ4‐thia‐1,6‐diazapentalenes5, a new typ of thiadiazapentalenes anellated with a heterocyclic ring system. The structure of5was evidenced by IR, UV,1H‐,13C‐n.m.r. spectral data and supported by elemental analysis. By means of15N‐ and13C‐n.m.r. spectroscopy the synthesized thiadiazapentalenes were found to