UO2, both pure and doped with tri-, quadri-, and penta-valent ions, has been labelled with rare gas by means of ion bombardment and reactor irradiation. The gas release at low gas concentrations occurred by a mechanism little or not dependent on self-diffusion of either oxygen or uranium. At high gas concentrations, retarded release of gas occurred. This was shown by transmission electron microscopy to coincide with formation and annealing of radiation-induced bubbles and dislocation loops.