In alkoxyselenenylation reactions of alkenes intermediate seleniranium ions 1 are formed. In competition experiments it was shown that the formation of these intermediates is reversible. The seleniranium ions of type 20(+) formed by addition of chiral selenium electrophiles to alkenes are the decisive intermediates in the asymmetric methoxyselenenylation reaction. Their stabilities are strongly dependent on the strength of the selenium-heteroatom interaction. This was shown experimentally, because an independent method has been used for the synthesis of different diastereomeric seleniranium ions. Furthermore, calculations have been carried out to determine the relative stabilities of the diastereomeric seleniranium ions 20(+). The results obtained from the calculations support the experimental findings.