Ammonia-water mixtures have been used as working fluids in absorption-refrigeration cycles for several decades. Their use;as multi-component working fluids for power cycles has been investigated recently. The thermodynamic properties required are known or may be calculated at elevated temperatures and pressures. We present a new method for these computations using Gibbs free energies and empirical equations for bubble and dew point temperature to calculate phase equilibria. Comparisons of calculated and experimental data show excellent agreement.