To facilitate simulation, design, and optimization of chilled ammonia processes for CO 2 capture, we develop a thermodynamicmodel for theNH 3-CO 2-H 2Osystem with the electrolyte NRTL activity coefficientmodel. The thermodynamic model explicitly accounts for the solution chemistry which includes dissociations of H 2O, NH 3, and CO 2, 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 NH 3-H 2O binary, solid-liquid equilibrium of the NH 4HCO 3-H 2O binary, and vapor-liquid equilibrium and speciation of the NH 3-CO 2-H 2O ternary. The model is further validated with additional VLE, speciation, heat capacity, and heat of solution data for the NH 3-CO 2-H 2Osystem. Overall the model satisfactorily represents the thermodynamic properties of the NH 3-CO 2-H 2O system with temperature up to 473 K, pressure up to 7 MPa, NH 3 concentration up to 30 wt %, and CO 2 loading up to unity.