A new empirical model is proposed for estimating the amount of volatile iodine in an aqueous phase. The volatile iodine concentration is estimated for highly irradiated CsI solutions in which the pH of the solution changes. The reaction of CsI solution with water radiolysis products is not balanced because radiolysis products are continuously produced under irradiation. Thus the kinetic of the chemical equation is important to determine iodine behavior in a CsI solution. An empirical model for the kinetic equation including the oxidation and reduction reaction is proposed. The proposed model was validated with a wide range of experimental data. A comparison of the experiments and predictions by the model indicated that the predicted volatile iodine from CsI solution with a concentration of 10−3 to 10−4 M was in good agreement. For 10−5 M CsI solution, the predicted iodine concentration was much smaller than experimental data due to the fact that I− was rapidly converted to IO3−.