Reactions of methyl(vinyl)iodonium ion and the &bgr;-substituted derivatives as well as divinyliodonium ion with chloride ion were examined theoretically by ab initio MO (MP2) calculations at the double-zeta (DZ) + d level. Interaction of the iodonium ion1with Cl-leads to chloro-&lgr;3-iodane2. Transition states for the SN2, ligand-coupling substitution (LC), and &bgr;-elimination (&bgr;E) are found for reactions at the vinyl group. The barrier to LC is usually the lowest in the gas phase, but relative barriers to SN2 and to &bgr;E change with the substituents. Effects of solvent were evaluated by a dielectric continuum model and found to be large on SN2 but small on LC. The experimental observations in solution that the SN2 is the most facile for the &bgr;-methyl-substituted1while it is retarded by the &bgr;-tert-butyl and &bgr;-chloro substitutions are reproduced by the calculations that take medium effects into account.Key words: vinyliodonium salt, polyvalent iodine, vinylic SN2 reaction, ligand coupling, ab initio MO calculation.