Mixtures of water and ammonia play an important role in absorption refrigeration cycles and have received attention as working fluids in modern power generation cycles. For design and simulations during the development of any application, the thermodynamic properties have to be known accurately. Measurements of available thermodynamic data are compiled and summarized. The data sets are compared, using a Helmholtz free energy formulation. Recommendations are given for which data sets are suited to serve as a basis for an equation of state formulation of the thermodynamic properties of{water+ammonia}.Gaps in the database are shown to give experimenters orientation for future research. ©1998 American Institute of Physics and American Chemical Society.