A method to expel radioiodine from a spent-fuel solution is important for iodine control in reprocessing plants. Many authors have investigated the procedure without considering the influence of other fission products on that procedure. The present work studies the behavior of iodine in a simulated spent-fuel solution containing fission products. When an iodide (1 mg I-) is put into a simulated spent fuel-3.4 M HNO3 solution (100 ml) at 100°C, it is 93.3 to 98.5% volatilized as I2, depending on the carrier gas, the presence of NO2, and the solute concentration. Colloidal iodine constitutes a significant part of the nonvolatile iodine species in this solution, whereas is predominant in a similar solution without fission products. The colloidal iodine varies from 0.4 to 2.9% of the initial iodine, depending on the foregoing experimental conditions. The colloidal iodine consists of such iodides as Pdl2 and Agl, which do not react with NO2 but are decomposed by such iodates as KIO3 and HIO3. Besides acting as carrier , these iodates are able to dissolve the colloid by oxidizing its iodine to I2. A high concentration is required to minimize the colloidal iodine. Increased HNO3 concentration (e.g., 6.1 M) increases the proportion of . The presence of NO2 increases the amount of colloid. Bubbling the solution with a N2 flow retards the formation of the colloid, probably because it prevents the aging of the colloid. Expelling >99% of the iodine from the solution requires additional , besides the action of NO2. These results indicate that the chemical reactions of fission products with iodine can interfere with the volatilization of iodine from the dissolver.