Thermodynamic modelling was performed for O-Th and O-Np systems to assess the phase stability of the MO2-X phase together with the data in the literature for O-Pu, O-U, and O-Zr systems. Firstly, thermodynamic modelling was demonstrated for O-Th and O-Np systems with thermodynamic data and phase diagram information available in the literature. The assessed data reproduced the general feature of the system with respect to the phase diagrams of the both systems and the oxygen potential in the ThO2-X and NpO2-X phases. Secondly the phase stability of the MO2-X phase was assessed using the obtained sets of data together with those for O-Pu, O-U, and O-Zr. The phase stability of the MO2-X phase was discussed with respect to the deviation from the ideal solution in oxygen potential and Gibbs free energy. (C) 2003 Elsevier Science Ltd. All rights reserved.