Grain growth was investigated in a UO2sinter of 94%) theoretical density over the temperature range 1555° to 2440°C. The results were in close, but not exact, agreement with a theoretical expression describing grain growth with a poly‐crystalline matrix. For the material studied the mean grain diameter D (μm) after annealing for t hours at a temperature T (°K) was given by the equationwhere D0and K0are, respectively, the initial grain size and a proportionality constant. Uranium metal was found in all specimens annealed above 2000°C. This was taken as evidence that the UO2lattice can be oxygen‐deficient at high temp