In the present study, new experimental equilibrium data are reported on the solubility of carbon dioxide into aqueous solutions of N-methyldiethanolamine (MDEA) and piperazine (PZ) over a wide range of conditions. These data not only include CO2 solubilities and their corresponding partial pressures, pH, and conductivities, but also a limited number of liquid speciation data obtained using NMR spectroscopy. The present data, and other data reported in the literature, were correlated with the Electrolyte Equation of State, as originally introduced by Furst and Renon. The final model was able to describe the thermodynamics of the quaternary CO2-PZ-MDEA-H2O reasonably well over a wide range of experimental conditions. (C) 2009 Elsevier Ltd. All rights reserved.