Tungsten (W) concentrations in fluids in equilibrium with crystalline tungsten oxide are used to improve constraints of thermodynamic parameters for W solubility. W species in the hydrothermal fluids are further characterized by X-ray spectroscopy. Improved thermodynamic properties for a set of W fluid species are provided that cover a wide range of fluid compositions, necessary for understanding and describing the complex processes of W enrichment and mineralization in hydrothermal systems.