A thermodynamic model about the absorption of CO2 by a solution of methyl diethanol amine (MDEA) and water was established with the electrolytic-NRTL (non-random two-liquid) equation for the calculation of activity coefficients of components in the solution. The gas absorption solubility data was used to obtain the parameters in the electrolytic-NRTL equation by regression and to directly predict the absorption enthalpy.