The present study provides data on the effect of total neutron dose and gas concentration on mobility and release of fission gases in UC, a question that is important for the swelling problem. UC powders with a carbon content of 4.68% were irradiated at reactor ambient temperature to doses between 1.5 × 1014and 1.5 × 1019nvt. The release of fission xenon was measured at a constant temperature of 1000C°. With increasing dose, a pronounced decrease with dose in the apparent diffusion coefficient was observed indicating trapping (i.e. gas-damage or gas-gas interactions). The kinetics of the release and the nature of the traps are discussed. The results are confirmed by additional data using UC single crystals that were labeled with xenon to different concentrations using ion bombardment techniques. The combined results yield a consistent picture of fission gas mobility and trapping in UC.