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The interaction of iodine species with ""Co-irradiated extractants of 30% TBP/70,% ?I-dodecane was examined to infer the behavior of iodine downstream of the dissolution step in reprocessing. When a simulated spent-fuel solution (in which the main iodine species was colloidal iodine) was contacted with the extractant with irradiation dose of 2.8xl@C/kg or less a t 25"C, 70 to 80% of iodine was transferred to the organic phase together with lOO$& of uranium. About 5 0 X of the iodine was retained therein during back-extraction of uranium by water a t 60°C and transferred to aqueous phase in the solvent cleaning by alkaline solution. Only 4 to 5 % of iodine was finally fixed in the organic phase. When the extractant was irradiated to 103C/kg or more, a "third phase" appeared in extraction step and the majority of iodine (about 6506) was sorbed in this phase. Over 90% of elemental iodine (I r) in a UO,(NO,),-HNO, solution was extracted to the organic phase regardless of irradiation dose of the extractant and half of it remained therein after solvent cleaning. Iodate (lo3-) in the same solution was extracted increasingly with increasing irradiation dose of the extractant.