This work reports the thermodynamic modelling assessment of the rather complex Cs-Mo-O system, which is key for the understanding of fission products chemistry in oxide fuelled Light Water Reactors (LWRs) and next generation Sodium-cooled and Lead-cooled Fast Reactors (SFRs and LFRs). The model accounts for the existence of the ternary molybdates Cs2MoO4 (α and β), Cs2Mo2O7 (α and β), Cs2Mo3O10, Cs2Mo4O13, Cs2Mo5O16, and Cs2Mo7O22, for which sufficient structural and thermodynamic information are available in the literature. These phases are treated as stoichiometric in the model. The liquid phase is described with an ionic two-sublattice model, and the gas phase as an ideal mixture. The optimized Gibbs energies are assessed with respect to the known thermodynamic and phase equilibrium data in the Cs2MoO4-MoO3 pseudo-binary section. A good agreement is generally obtained within experimental uncertainties. The calculated vapour pressures above Cs2MoO4 (solid and liquid) are also compared to the available experimental data. Finally, isotherms of the Cs-Mo-O ternary phase diagram are calculated at relevant temperatures for the assessment of the fuel pin behaviour in LWRs, SFRs and LFRs.@en