The equilibrium of CO2 solubility and species concentration data for the absorption of CO2 in the aqueous solutions of Methyl-di-ethanolamine (MDEA), Piperazine (PZ) and their mixture (MDEA+PZ) were analyzed using Matlab software. A thermodynamic model was developed to predict the Vapor liquid equilibrium of CO2 in aqueous mixtures of MDEA/PZ. The electrolyte-nonrandom two-liquid (e-NRTL) model was used to build the vapor-liquid equilibrium model, which defines equilibrium behavior of the solution. This model was applied to correlate the species concentration in the solutions of MDEA, PZ and their mixtures using default interaction parameters reported in the literature. It was observed that this model gave a good estimation of CO2 loadings over a wide range of operating conditions for both single and mixed amine solutions. The model has been used to estimate the species concentrations and CO2 solubility in aqueous DEA/PZ [1]. The application of this model is new to this system, i.e. of CO2-MDEA-PZ-H20. In this work, the model was further extended to predict the effect of piperazine on CO2 loading for carbonated aqueous solutions of MDEA/PZ systems. The addition of PZ to MDEA as an activator increased the solubility of CO2 under low and high range of CO2 partial pressures and temperatures. The species concentrations, activity coefficients, and pH of the CO2 loaded solutions were studied, and the model predicts amine volatility as well. The model was validated using the experimental data generated from our previous work and data available in the open literature. The results of the current model are in an acceptable degree of agreement with the experimental data of CO2 solubility reported in previous work for this system. (C) 2016 Published by Elsevier B.V.