A thermodynamic assessment of the urea− biuret− water ternary system and its binary subsystems has been made. The Margules model has been applied for the description of excess Gibbs energies of binary solutions, and the properties of ternary solutions have been assessed by using the Muggiani method. Thermodynamic properties of (NH2 CO)2 NH· (NH2 )2 CO (I) and (NH2 CO)2 NH· 0.7H2 O (II) have been estimated from the condition of equilibrium between the solution and stochiometric phases. The liquidus surface and isothermal sections of the phase diagram of the urea− biuret− water system at the temperature interval T = (268 to 373) K have been calculated. The comparison of calculated and experimental data is discussed.