Abstract: This paper reviews the author's own works related to the research on the mechanism of electrophilic addition to alkenes of sulfenyl halides, and puts emphasis upon the chemistry of thiiranium ions, which have been postulated as intermediates in the above addition reaction. The bridged thiiranium ions can be found as intermediates in some instances and can be prepared at low temperature, but of which the open ions are favored under the influence of benzylic or tertiary carbocation formation. Both stereo-and regio-chemistry ar dominated by the equilibrium between the bridged and open ions.