A physicochemical model developed in earlier work for representing H2S and CO2 solubility in aqueous solutions of monoethanolamine (MEA) and diethanolamine (DEA) was extended to include the mixtures of methyldiethanolamine (MDEA) with MEA or DEA. The framework of the model is based upon both liquid-phase chemical equilibria and vapor-liquid (phase) equilibria. Activity coefficients are represented with the electrolyte-NRTL equation treating both long-range electrostatic interactions and short-range binary interactions. Adjustable binary interaction parameters of the model were fitted on binary and ternary system MDEA data reported in the literature. The solubility of CO2 in aqueous mixtures of MDEA with MEA or DEA was measured at 40 and 80 °C over a wide range of CO2 partial pressures. Representation of the data by the model is good, especially at low to moderate acid gas loadings.