The separation of radioiodine from water by multistage isotope exchange between iodide and molecular iodine in a heterogeneous liquid system under both static and dynamic conditions has been studied in two model arrangements. Equations were derived which enable the calculation of the theoretical separation efficiency in dependence on both the volume concentration and concentration of the carrier of radioiodide in water. It was found that under the same conditions, the calculated values are in a satisfactory agreement with experimentally determined values of the separation efficiency.