The fission-gas release from single-crystal UO2 has been studied during irradiation at temperatures from 400°C to 1300°C and neutron fluxes from 1 × 1013 to 4 × 1013. The fractional gas release (rate of release/rate of production) was found to decrease with burnup and with increase in fission rate. Fission-gas release was independent of temperature below 600°C but increased exponentially with higher temperatures. From the proportions of different isotopes in the fission gas, it was concluded that a knock-out process controlled the low-temperature gas release. The high-temperature release, once thought to be by diffusion, is now postulated to be controlled by a trapping mechanism.