The solubility of tungsten trioxide solid and the speciation of tungsten in NaCl-bearing solutions have been investigated through experiments conducted at 250, 300, and 350 degrees C under vapour-saturated water pressure. Based on the results of these experiments, the solubility of tungsten trioxide was controlled by temperature and pH, whereas the NaCl concentration did not affect the solubility except through its influence on the ionic strength of the solution. Two tungsten species were found to be present in the solutions, namely H2WO40 at low pH and HWO4- at higher pH. These two species formed via the reactions WO3 + H2O = H2WO40 and WO3 + H2O = HWO4- + H+, respectively. The logarithms of the equilibrium constants for these reactions are -5.18 +/- 0.26, -4.97 +/- 0.25, -4.69 +/- 0.10, and -7.91 +/- 0.30, -7.67 +/- 0.29, -7.52 +/- 0.18 for 250, 300, and 350 degrees C, respectively. In addition, the logarithms of the first and second association constants of H2WO40 were determined to be 2.72, 2.71, 2.83, and 5.59, 6.49, 8.07 for 250, 300, and 350 degrees C, respectively. These values indicate that H2WO40 is only important at low pH values (<2.8), and that HWO4- is the dominant tungsten species at pH conditions commonly encountered in nature. The data obtained in this study were used to model the solubility of scheelite and ferberite. This modeling indicates that tungsten concentrations are highest at high temperature in solutions with high salinity, low contents of calcium and iron, and either very low or high pH. The opposite is true for tungsten mineral precipitation from a fluid. (C) 2019 Elsevier Ltd. All rights reserved.