A procedure is outlined that allows determination of the hydration state, charge and chemical stoichiometry, as well as the apparent standard molal Gibbs free energy of formation, of aqueous species from solubility measurements in fluid mixtures. Also described is a hydrothermal apparatus capable of obtaining the necessary solubility data in fluid mixtures. The apparatus was used to determine quartz solubilities in supercritical CO2-H2O and Ar-H2O mixtures. Results of this investigation along with other values reported in the literature are consistent with a stoichiometry of the dominant aqueous silica species of Si(OH)4 · 2H2O in the supercritical region of H2O. Knowledge of the hydration number of aqueous silica was used to predict silica concentrations in CO2-H2O mixtures in the system CaO-MgO-SiO2-HCl-CO2-H2O at 2 kbar and 450°C.