The oxygen potentials at 1,000 and 1,300°C and the lattice parameters of UO2 fuels with soluble fission product elements (Zr, Ce, Pr, Nd, Y), simulating high burnup of up to 10a,o have been measured by means of thermogravimetry and X-ray diffraction. The oxygen potentials for (U, FP)O2+x fuels are higher than pure UO2+x; at a given composition and increase positively with increasing simulated burnup. They can be represented as a function of the mean uranium valence at compositions of 0/M>2.01. The lattice parameters of stoichiometric (U, FP)02.00 fuels decrease linearly with simulated burnup, and can be expressed as a (pm) = 547.02–0.1225, where B is burnup in a.o