A knowledge of the thermodynamic properties of UO2 at temperatures in the region 1 500-3 100 K is of importance in reactor safety calculations, yet there are relatively few detailed experimental data available. In particular the major question of whether Frenkel disorder occurs in UO2 at high temperatures has been unanswered until now. A new high temperature furnace has been purchased by Harwell for work at temperatures in this region, and a series of experiments has been carried out involving diffraction, quasielastic diffuse and inelastic neutron scattering from single crystals of UO2 and ThO2. These have been backed by experiments in the lower temperature range to 2 500 K at I.L.L. Details of the Harwell furnace, and methods used for temperature measurement and encapsulation of the crystal samples are given, together with some examples of the principal results. These results show unambiguously, for the first time, that oxygen Frenkel disorder does occur in UO2 above 2 000 K.