The behavior of He and Xe implanted into UO2 single crystals is studied by in situ TEM experiments before and after annealing up to 700 °C. TEM micrographs show that annealing induces the formation of noble-gas bubbles in both cases. However, the size (25 nm for He and 3–5 nm for Xe) and the nucleation temperature (600 °C for He and 400 °C for Xe) of bubbles depend on implanted species. These results are explained by the radiation damage produced by ion implantation (different by a factor of 100 for the two elements) and the diffusion mechanisms involved in each case.