The Clegg-Pitzer equations with some simplifications are used to correlate the vapor-liquid equilibrium data for the CO2-methyldiethanolamine-sulfolane-H2O and H2S-methyldiethanolamine-sulfolane-H2O systems. The interaction parameters determined from data for the binary, ternary, and quaternary systems can be used to predict the quinary mixed solvent system without any additional parameters. The model is applied to the CO2-H2S-methyldiethanol-amine-sulfolane-H2O system containing three neutral solvents (methyldiethanolamine, sulfolane, and H2O), two neutral solutes (CO2 and H2S), and three ionic species (MDEAH(+), HCO3-, and HS-) with three main chemical equilibria simultaneously involved.