An equation of state (EOS) explicit in Helmholtz free energy was developed to calculate the PVTx and vapor-liquid phase equilibrium properties of the NH3-H2O fluid mixture. This EOS, where four mixing parameters are used, is based on highly accurate EOSs for the pure components (H2O and NH3) that NIST recommends and contains a simple generalized departure function presented by Lemmon and Jacobsen (1999) [38]. Comparison with thousands of reliable experimental data available indicates that the EOS can calculate both vapor-liquid phase equilibrium and volumetric properties of this binary fluid system, within or close to experimental uncertainties up to 706K and 2000 bar over all composition range. The average absolute deviation is 0.68% in molar volume, and the average composition error of vapor phase and that of liquid phase except for those at the near-critical region are in general less than 0.03 and 0.07 in mole fraction, respectively. (C) 2015 Elsevier B.V. All rights reserved.