A thermodynamic model for the system NH3-CO2-H2O-urea is developed as a supporting program of a urea synthesis reactor simulation module. The model covers a wide range of composition and temperature and can be used to predict the behavior of the system at and removed from urea synthesis conditions. Calculated equilibrium phase compositions and vapor pressures at different temperatures are in very good agreement with published experimental data. When used as thermodynamic support of the reactor simulation module, the proposed thermodynamic model yields good agreement with production-scale plant data. Calculated partial molal enthalpies for all components in the gas and liquid phases lead to a satisfactory prediction of local equilibrium temperatures. The solution algorithm for the nonlinear system of phase and chemical equilibrium equations is simple and robust. A multiple-step parameter estimation strategy is adopted to regress published experimental data.