New experimental results are presented for the influence of sodium sulfate on the total pressure above liquid mixtures of (H2O + CH3OH + CO2) at temperatures of about 313.7, 354.4, and 395 K, total pressures up to about 10 MPa, mole fractions of methanol in the binary solvent mixture (water + methanol) of about 0.055 and 0.25, and salt molalities of about 0.92 and 0.15 mol/kilogram of water + methanol, respectively, always below the solubility limit of the salt in the gas-free solvent mixture. An extension of Pitzer's molality scale based model for the Gibbs excess energy of aqueous electrolyte solutions to mixed-solvent electrolyte solutions is successfully applied to predict/correlate the new gas solubility data.