The compression behaviour of a solid solution of ThO2 and hyperstoichiometric UO2 was examined up to 11 GPa hydrostatic pressure using x-ray diffraction. We observed a distortive phase transition above 3 GPa from fluorite-type to a structure isotypic with t-zirconia. The transition is tentatively explained by merging of local tetragonal UO(2+δ) clusters into an itinerant structural distortion. The bulk modulus increases from 162 ± 1 to 199 ± 4 GPa in the tetragonal phase by stiffening of the c-axis compression.