An inter-laboratory comparison has provided enhanced confidence in a novel out-reactor method that has been developed for investigating the migration behaviour of fission products in oxide nuclear fuels. Changes in the distribution of ion-implanted fission products caused by thermal annealing or radiation damage are precisely determined using high-performance secondary ion mass spectrometry. Accurate measurements of iodine thermal diffusion in stoichiometric UO2on single-crystal as well as polycrystalline ceramic samples over the temperature range 1200DC to 1650DC for periods ranging from 1 h to 24 h have yielded a refined Arrhenius relationship. An increase in both the solubility and diffusion rate of iodine by two orders of magnitude compared to stoichiometric fuel was found when the annealing was performed at 1400DC under slightly oxidizing conditions sufficient to achieve a nominal O-U ratio of 2.02. These effects are consistent with the increase in the population of uranium lattice vacancies predicted by a thermodynamic model for the defect structure of UO2+x.