The solubilities of solid oxides in the vapor phase of water are considered as the gas phase chemical reactions of metal oxides and water with the formation of hydroxides of metals and their hydrates. The necessary thermodynamic properties of hydroxides and their hydrates in the ideal gas state are obtained by quantum chemical calculations or in a few cases are evaluated from available experimental data. The values of the fugacity coefficients (phi(infinity)(2)) of components of gas mixtures are estimated by the empirical relation ln phi(infinity)(2) = k . ln phi(1)*, where phi(1)* stands for the fugacity coefficient of pure water and the parameter k is evaluated by counting the groups (OH, H2O, O) of the molecule of a dissolved compound, which can form hydrogen bonds with water molecules. Considered examples include the calculations of the steam solubility of MoO3(s), ZnO(s), and Cu2O(s) at 573-1273 K and up to water density of 300 kg m(-3) in comparison with available experimental data. It was found that the speciation of Mo(VI) is absolutely dominated by a single form MoO2(OH)(2), while in the Zn(II) and Cu(I) steam mixtures both hydroxides and their hydrates contribute appreciably to the material balance of metals. There is a clear tendency to the dehydration of dominating forms with an increase of temperature. The agreement of the experimental and calculated solubility values can be termed satisfactory (for Cu2O(s)-H2O at least for temperatures of 623.2 and 673.2 K), considering the difficulties of gas-phase solubility experiments, uncertainties of the data based on quantum chemical calculations, and the simplicity of the model to predict phi(infinity)(2) values. (c) 2012 Elsevier B.V. All rights reserved.