Sulfenium and selenenium ions undergo a stereospecific transfer from the corresponding three-membered ring species ("-iranium ions") to unactivated alkenes with varying facility. The thiiranium and seleniranium hexafluoroantimonates could be generated by treatment of the corresponding chloro sulfides or selenides with silver hexafluoroantimonate, followed by removal of the silver chloride by filtration. Clean (1)H, (13)C, and (77)Se NMR spectra could be recorded for these species. Treatment of the S-phenylthiiranium ion with an alkene leads to a slow transfer of the sulfenium group at 0 degrees C. However, the S-methylthiiranium ion did not transfer the sulfenium group, even at room temperature. On the other hand, both the Se-phenyl- and Se-butylseleniranium ions transferred the selenenium moiety instantaneously at -70 degrees C. By measuring the equilibrium position for these transfers from both directions, the relative stability of the 1-phenylseleniranium ions could be established: cis-tetramethylene < trans-2,3-dipropyl approximately trans-2,3-diisopropyl < cis-hexamethylene.