Abstract Reaction of o-alkynylbenzothioamide with molecular iodine was investigated. Unique benzo [c] thiophen-1-aminium iodide was obtained when the tertiary thioamide was used as a reactant. The reaction proceeded efficiently in less polar solvent, whereas the corresponding product was also obtained in a polar solvent. Single crystal X-ray analysis revealed that the benzo [c] thiophen-1-aminium structure possesses a resonance structure ...