Accurate modeling of the solubility behavior of CO_2 in the aqueous alkanolamine solutions is important to design and optimization of equipment and process. In this work, the thermodynamics of CO_2 in aqueous solution of N-methyldiethanolamine(MDEA) and piperazine(PZ) is studied by the electrolyte non-random two liquids(NRTL) model. The chemical equilibrium constants are calculated from the free Gibbs energy of formation, and the Henry’s constants of CO_2 in MDEA and PZ are regressed to revise the value in the pure water. New experimental data from literatures are added to the regression process. Therefore, this model should provide a comprehensive thermodynamic representation for the quaternary system with broader ranges and more accurate predictions than previous work. Model results are compared to the experimental vapor-liquid equilibrium(VLE), speciation and heat of absorption data, which show that the model can predict the experimental data with reasonable accuracy. Accurate modeling of the kinetic behavior of CO_2 in the aqueous alkanolamine solutions is important to design and optimization of equipment and process. In this work, the thermodynamics of CO_2 in aqueous solution of N-methyldiethanolamine (MDEA) and piperazine (PZ) is studied by the chemical non-random two liquids (NRTL) model. The chemical equilibrium constants are calculated from the free Gibbs energy of formation, and the Henry’s constants of CO_2 in MDEA and PZ are regressed to revise the value in the pure water. New experimental data from literatures are added to the regression process. the model should provide a comprehensive thermodynamic representation for the quaternary system with broader ranges and more accurate predictions than previous work. and heat of absorption data, which show that the model can predict the experimental data with reasonable accuracy.