[en] The chemical form of radioiodine and the physicochemical behavior of each individual form of radioiodine in the marine environment is still not so well known, owing partly to the lack of the appropriate analytical method. To study on these problems, the authors have first developed the radiochemical method of analysis for the individual forms of radioiodine. The method is based on the solubility of I² and CH³I in carbon tetrachloride or toluene and on the difference of isotopic exchange behavior between I^- and IO^3-. By shaking the sample sea water with carbon tetrachloride, I* ² and CH³I* can be extracted, if present. Among these, only I* ² can be back-extracted with sodium sulfite solution. I*^- and I*O^3- can be determined by using the following isotopic exchange system: I*^- (aq.) + I² (org.) reversible I^- (aq.) + I* ² (org.). When the pH of the aqueous layer is adjusted to 4, only I*^- exchanges with I², while I*O^3- does not. Therefore, the activity of the organic layer at the exchange equilibrium corresponds to that of I*^-. At pH 1, I*O^3- reacts with I^- carrier to be converted to form, and the activity of the organic layer corresponds to the total activity of I*^- + I*O^3-. For organic solvent, the equi-volume mixture of carbon tetrachloride and toluene is found to be the most convenient among those tested. (author)