Reactions of certain 3-bromothio-1,2–dithiolium bromides with primary amines produce isothiazoline-5-thiones. These compounds form adducts of varying stability with acetylenic reagents. Comparison of their reactivity with the isomeric isothiazoline-3-thiones indicates that while the former react rapidly to form only monoadducts, the latter react more slowly to form monoadducts which react more rapidly to form diadducts.