The solubility of oxides of metals and metalloids in water can be considered as a reaction of hydration with the formation of hydroxides species, like Al(OH)(3)center dot H2O, Si(OH)(4), B(OH)(3), etc. Calculation of solubility requires, along with the equilibrium constant of the dissolution reaction in the ideal gas state, knowledge of the fugacity coefficients, phi(infinity)(2), for aqueous species. This work describes simple ways of estimating fugacity coefficients for such species in water of various pressures/densities, based on the stoichiometry of hydroxides or the corresponding-states correlation. The method is verified by satisfactorily predicting the solubility of quartz, SiO2, at 647-1773 K, and the pressure dependence of solubility of corundum, Al2O3, at 973 K in water up to 2000 MPa. A few experimental data points, if available, may significantly improve the accuracy of the proposed method.