[en] Zirconia doped with MgO, CaO, Y₂O₃ and rare earth oxides is a remarkable material due to its technological importance, basic scientific interest and superior synthetic properties. Knowledge of phase diagrams and thermodynamic properties is of great importance in the production and application of zirconia-based ceramic materials. Recently, thermodynamic modeling of zirconia-bearing systems has received considerable attention because of the difficulties to construct zirconia-based phase diagrams and thermodynamic properties by means of experimental methods. The modeling for binary systems can be used to check for internal consistency between thermodynamic and phase diagram data and to provide optimal thermodynamic parameters. The predicted ternary phase diagrams based on the description of the binary systems give a prospect of phase relations in the ternary systems, which have not been established experimentally and have not made it possible to design efficient experiments. The same is true of going from ternary systems to a quaternary one and so on. In the laboratory the authors are currently engaged in a thermodynamic modeling of the ZrO₂-CeO₂-REO_1.5 (RE = Rare Earth) systems with a view to develop a database for the zirconia-based systems. The present article reports on the thermodynamic analysis of the ZrO₂-CeO₂ system. The thermodynamic modeling of the C_ss (solid solution of cubic ZrO₂ and CeO₂) and T_ss (solid solution based on tetragonal ZrO₂) equilibrium in the ZrO₂-CeO₂ system has been conducted, firstly, by one of the authors using two-term lattice stability values. In view of the fact that the two-term lattice stabilities are now out of date and there has been abundant information on the ZrO₂-CeO₂ system since the previous assessment, a new assessment of this system is necessary