A thermodynamic model for the solubility of carbon dioxide in aqueous solutions of (N-methyldiethanolamine (MDEA)+ piperazine (PIPH2)) is presented. The model combines and extends previous models for the solubility of CO2 in aqueous solutions of the single amines and also accounts for the volatility of both amines. Model parameters are estimated from previously published experimental results for the solubility of CO2 in aqueous solutions of both amines (for MDEA molalities up to 8 mol kg(-1) of water and for PIPH2 molalities up to 4 mol kg(-1) of water, temperatures between 313 and 393 K, and pressures up to about 10 MPa). (C) 2010 Elsevier B.V. All rights reserved.