The solubility of NH3 in multiple systems under high pressure is of extreme importance to the new integrated ammonia and urea process. This paper proposes a new method for calculating the vapor-liquid equilibria for aqueous ammonia solutions under high pressure. Based on the experimental data of the binary system NH3-H2O in the high concentration range

a model has been established for the activity coefficients of NH3 and H2O to describe the non-ideality of the liquid phase; and the modified SIRK equation of state for polar compounds has been used to describe the nonideali-ty of the vapor phase at high pressures. The reliability of this model has been examined in the light of the experimental date of the solubility of NH3 under high pressure in the systems NH3-H2O-N2

NH3-H2O-N2-H2-Ar-CH4

and NH3-H2O-N2-H2

the last from Cuerreri et al. Both the accuracy of calculation and the concentration range for applicability of this model have been found to surpass those of Curreris model.