A thermodynamic model was developed for representing vapor-liquid equilibria (VLE) of the CO2-H2S-MEA-MDEA-water system. The model accounts for chemical equilibria in the liquid phase and physical equilibria between the liquid and vapor phases. Activity coefficients are represented by the electrolyte-UNIQUAC equation. The present extension uses an ion-pair interaction approach and satisfies both the principles of like-ion repulsion and local electroneutrality. Contributions from Iona-range ion-ion interactions are represented by a Debye-Huckel formula suitable for mixed solvents, water and alkanolamines. Adjustable parameters of the electrolyte-UNIQUAC equation, representing short-range binary interactions, were determined by data regression using binary, ternary, and quaternary system VLE data. Predicted H2S and CO2 vapor pressures are in good agreement with the reported experimental data for aqueous solutions of a single acid gas as well as mixtures of H2S and CO2 in MEA and MDEA and their mixtures in the temperature range 25-120 degreesC.