A thermodynamic assessment of the U-Ce-O system was performed by combining the existing Calphad assessments of the U-O and Ce-O binaries. A compound energy formalism representation for the fluorite U1-yCeyO2 +/- x and a partially ionic two-sublattice liquid model for U-Ce-O melt were developed to describe the ternary solutions. Oxygen potentials for U1-yCeyO2 +/- x for 0.05, 0.20, 0.30, and 0.50 Ce metal fractions were measured from thermogravimetric analysis and used, along with other thermodynamic data reported in the literature, to optimize the adjustable parameters of the models for U1-yCeyO2 +/- x and the U-Ce-O ternary liquid. In addition, the thermochemical behavior of U1-yCeyO2 +/- x and U1-yPuyO2 +/- x was compared in order to assess the suitability of using Ce as a surrogate for Pu in MOX fuel. (C) 2015 Elsevier B.V. All rights reserved.