To facilitate simulation, design, and optimization of chilled ammonia processes for CO2 capture, we develop a thermodynamic model for the NH3-CO2-H2O system with the electrolyte NRTL activity coefficient model. The thermodynamic model explicitly accounts for the solution chemistry which includes dissociations of H2O, NH3, and CO2, formation of ammonium carbamate, and precipitation of ammonium bicarbonate. The electrolyte NRTL activity coefficient model parameters are identified by fitting against selected experimental data for vapor-liquid equilibrium, heat of solution, and heat capacity of the NH3-H2O binary, solid-liquid equilibrium of the NH4HCO3-H2O binary, and vapor-liquid equilibrium and speciation of the NH3-CO2-H2O ternary. The model is further validated with additional VLE, speciation, heat capacity, and heat of solution data for the NH3-CO2-H2O system. Overall the model satisfactorily represents the thermodynamic properties of the NH3-CO2-H2O system with temperature up to 473 K, pressure up to 7 MPa, NH3 concentration up to 30 wt %, and CO2 loading up to unity.