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 °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 °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 °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.