Aqueous fluids are the most effective mass transport agents on the Earth and the reactive fluid flow through the lithosphere is often responsible for metal mobilization, transport and deposition in the form of economic enrichments. The fluid-mediated mass transfer as well as hydrodynamic computational modeling require uniform and consistent basis of thermochemical and transport properties of individual constituents and these serve as model or process constraints. Unlike for high-density liquid-like fluids, the thermodynamic data for solutes in low-density fluids are much more fragmentary and no uniform thermodynamic formalism exist to adequately represent the short-range hydration interaction and to address metal solubility, partitioning and speciation in low-density fluids and vapors. This project will develop a new thermodynamic model and dataset for aqueous species in low-density fluids. We will concentrate on evaluation of leading terms and functional forms representing variable structure and compressibility of the hydration shell. The new equation of state will be calibrated for representative solutes by available experimental and molecular-dynamics simulation data. This approach will extend capability of thermodynamic models to represent metal transport in low-density fluids, particularly during magmatic devolatilization and in expanding or boiling hydrothermal and geothermal systems.