A new approach is presented for calculation of activity coefficients in multicomponent aqueous electrolyte and non-electrolyte solutions. The approach is based on an integration of the Gibbs-Duhem equation. It is shown that using standard thermodynamic properties for the pure components and for the components at infinite dilution in water, it is possible to predict joint solubilities of both electrolytes and non-electrolytes. To illustrate the usefulness of the method proposed in this work, calculations of solubility are presented for the two-electrolyte systems H2O-NaCl-KCl, H2O-NaNO3-KNO3, H2O-CaCl2-NaCl, and for a system containing both an electrolyte and a non-electrolyte H2O-sucrose-NaCl. The solubility of carbon dioxide at different pressures in water containing various electrolytes [Ca(NO3)(2), NaCl, CaCl2,Na2SO4] is also calculated.