A model for the Gibbs excess energy of mixed-solvent (chemical-reacting) electrolyte systems is presented. The activities of solvent and solute species are calculated by applying a new extension of Pitzer's equation for the excess Gibbs energy of aqueous electrolyte solutions, which allows for solvent mixtures. The capability of the new model to simultaneously describe such properties as, e.g., the mean ionic activity coefficient and the solubility of a salt (for example, sodium chloride) in a binary solvent mixture (for example, methanol + water), as well as the influence of that salt on the vapor-liquid equilibrium of that binary solvent mixture, is tested. The new model gives an explanation for the "salting-out" and "salting-in" effects resulting, e.g., from the addition of a salt to a liquid mixture of volatile solvents. The new model can also be applied to describe the gas solubility in mixed solvents in the presence of electrolytes. The capability of the new model to describe the solubility of a single gas in a binary solvent mixture is tested by dealing with the solubility of carbon dioxide in aqueous solutions of methanol over the whole range of solvent composition, i.e., from pure water to pure methanol.