A thermodynamic model for describing the phase equilibrium for the system CO2 + methyldiethanolamine + water has been extended, allowing for the presence of piperazine. Model parameters are estimated from new experimental results for the solubility of carbon dioxide in aqueous solutions of piperazine (for piperazine molalities up to 4 mol per kilogram of water, temperatures between 313 and 393 X and pressures up to about 9.6 MPa). The model is tested by comparing predicted and experimental carbon dioxide solubilities in aqueous mixtures of both amines (2 molal MDEA + 2 molal piperazine, at 353 K, and pressures up to about 6 4 MPa).