Lithium, sodium, and caesium iodides are highly ionized in acetonitrile solution and have practically the same rates and activation energies (18 kcal. per mole) for their exchange reactions withn-butyl iodide in this solvent. There seems little doubt, therefore, that these exchange reactions occur between the organic iodide and iodide ion derived from the inorganic salt. Zinc and cadmium iodides, and iodine, are only slightly ionized in acetonitrile and exchange withn-butyl iodide with rates that are widely different from one another and from the rate of iodide ion exchange. The activation energies are 21.9, 20.6, and 17.5 kcal. per mole respectively. It is concluded that these exchange reactions involve predominantly complex ions.