A program has been conducted and is continuing at Battelle Memorial Inst. under the sponsorship of the United States Atomic Energy Commission to study the release of fission gas from UO2 and to relate the release to material properties, both chemical and structural. Work in the program has involved the preparation and characterization of sintered polycrystalline and fused single-crystal UO2 samples, the construction of a beam-tube loop facility for the irradiation of the UO2 under controlled temperature and atmosphere, and the collection and analysis of released fission gases during irradiation. Irradiation experiments have been conducted in purified helium-hydrogen atmosphere at temperatures from 500 to 1500 F, with fission gases continuously collected and analyzed for krypton and xenon during irradiation of natural uranium specimens. Irradiation exposures of the specimens have been in the region of 1018 nvt.