The desorption spectroscopy of trapped gas by low energy ion bombardment in sintered and single crystal UO2, enables us to estimate the threshold energy for entrapment, the trapping coefficient and its variation with both ion energy and implantation temperature and the activation energy for desorption. By comparing the desorption spectra from sintered and single crystal UO2 the surface effects are clearly illustrated for sinters and practically nonexistent in single crystals. The same conclusion could be drawn from fractional gas release experiments. From the entrapment studies of nobles gas ions in UO2, information on gas dissolution, diffusion and bubble formation processes could be derived. Recrystallization of near surface layers and gas bubble production induced by energetic ion bombardment at high doses, observed by TEM, have shown to be closely related to gas release stages.