Abstract Gamma radiolysis of iodine ions in aqueous solutions was investigated under high intensity gamma-rays (at 4.5 × 10 6 R/h) from 60Co at ambient temperature. The yields of radiation induced oxidation products, I 2 and IO 3 − were measured as a function of radiation dose, I − concentration and solution pH. The I 2 yield increased with increasing I − concentration and decreasing pH. The G(I 2) value is determined to be 0.2 in a pH 2 solution containing 10 −3 MI −. At an iodide concentration of 10 −5 M or less, the formation of I 2 diminished, whereas IO 3 − was found. Iodide was converted nearly quantitatively to IO 3 − hwern the initial I − concentration was 10 −6 M or less with a pH range from 3 to 9. The maximum G(IO 3 −) value is determined to be 0.001 in a pH 8 solution. A mechanism is proposed to explain the formation of IO 3 − under the experimental condition. This mechanism consists of successive oxidations of iodine with HIO as a primary intermediate. Experimental data have demonstrated that the formation of iodate in the BWR primary coolant is the result of radiation effects on radioniodine in the coolant. The mechanism would predict that the iodine in the BWR steam phase is probably the result of distillation of the volatile HIO intermediate.