Spent-fuel specimens (∼3 g each) with a burnup of 21 to 39 GWd/t were dissolved in 30 ml of 4 M HNO3 at 100°C, and the distribution of iodine and its chemical forms in the solution were studied. A small quantity of the iodine was conveyed to the insoluble residue (up to 2.3%), some remained in the fuel solution (up to 9.7%), and the balance was in the off-gas. Iodine was not deposited on the fuel cladding. Organic iodides were only ∼6.5% or less of the total amount of iodine in the off-gas. The fuel solution included iodine species that were difficult to expel by NO2 sparging alone (27 to 46% of the iodine in the solution). These species were ascribed to be the colloids of AgI and Pdl2. Io-date () was a rather minor iodine species in dissolution in ∼4 M HNO3. A thermochemical calculation also supports these results, indicating that the quantity of is ≦ 1.7 × 10−4% of the iodine fed to 4 M HNO3 and that the colloid of Agl can be formed when the concentration of I- is ≧ 5.3 × 10−10 M. For this calculation, the solubilities of Agl and PdI2 in water were measured: They are 6.5 × 10−8 and 6.3 × 10−7 M, respectively, at 90°C. According to supplemental experiments using a simulated spent-fuel solution and 1311, NOx sparging, previously proposed by earlier workers for expulsion of from the fuel solution, retards the rate of decomposition of the colloid. The thermochemical calculation indicates this is because the equilibrium concentration of I- in 3 M HNO3 is increased two orders of magnitude (1.7 × 102 times) by sparging with 10% NO2. The NO2 sparging brings the concentration of I- close to the solubility of Agl and suppresses the decomposition of its colloid into I2. Heating the solution without NO2 sparging was effective for the decomposition of the colloid, and the addition of excess amounts of accelerates its rate of decomposition.