3-Mercaptobenzo[b]thiophene (I) is transformed with assistance of bases, esp. of amines, into the benzo[b]thieno-annellated 1,2-dithiin II and not into the originally claimed \"dithioxo-thioindigo\" trans compd. Primarily, the formation of II from I involves oxidn. to the disulfide of I which may also be used sep. Conceivable rationalizations are discussed. II is accessible (in lower yield) by thionation of thioindigo with the aid of the Lawesson reagent. X-ray elucidation of II reveals a nonplanar structure of the cyclic disulfide with a dihedral C-S-S-C-angle of 53 Deg. Despite of the absence of any established chromophoric moiety, II is deeply red in the crystal and in soln., as it is representative for the \"undisturbed\" 1,2-dithiin system. Contrary to the usual behavior of the non-annellated 1,2-dithiins, II displays no spontaneous S extrusion under mild conditions, but only at high temps. and in the presence of sulfur-binding agents, leading to a benzo[b]thieno-annellated thiophene. Moreover, II may be reduced to the dithiol and regenerated exclusively from this by oxidn. Further characteristic reactions of II are described (e.g. to a sulfoxide, to a pyridazine, and to a di-Et thiophosphate).