A thermodynamic framework for representing chemical and vapor-liquid equilibria in acid gas-MDEA-water systems has been developed. The vapor-liquid equilibrium is calculated through a new EoS/G(E) model called the electrolyte-LCVM model. Model parameters either are taken from the literature (when they are available) or are determined in this work through the regression of binary and ternary vapor-liquid equilibrium data. Accurate predictions are obtained with the new model over a wide range of temperatures, pressures, MDEA concentrations, and loadings.