A thermodynamic model of the ammonia carbondioxide water urea system at urea synthesis conditions, that is, at t = (135 to 230) ° C, p = (3.5 to 45) MPa, L = nN /nC = (2 to 5.5) and W = ( nH2O n (NH2)2CO ) = nO /nC 2 = ( 0.75 to 1.2) was developed. A liquid phase was described by the UNIQUAC model including urea, ammonium carbamate, and ammonium bicarbonate as compounds; the gas phase was described by a virial equation of state. Bubble point pressures and carbon dioxide to urea conversion data were used for the model parameters optimization. The unique features of the model are the correct description of the saddle azeotrope and intensive use of existing thermodynamic data about constituents and binary subsystems vapor liquid equilibria data.