The fission-gas release from UO2 samples was studied during irradiation in the temperature range 150 to 2000° C. Both sintered UO2 and single crystals were studied. Below 600° C, recoil and knock-out are the mechanisms of release. Between 800 and 2000° C, excluding the evaporational escape above 1800° C, the fractional release follows an Arrhenius law where activation energy decreases with increasing density. The fractional release was found to be independent of neutron flux.