An accurate thermodynamic model is the primary element needed for the process simulation and optimization for CO2 absorption in aqueous amine solutions. In this work, the thermodynamic model was built in Aspen Plus, using the electrolyte nonrandom two-liquid (ENRTL) activity coefficient model to represent vapor pressure and heat capacity data, simultaneously, for amine, vapor-liquid equilibrium (VLE), excess enthalpy (H-E), and pKa data for amine/H2O, and CO2 solubility data for amine/CO2/H2O. The cyclic diamine 1-methylpiperazine (1MPZ) is a promising amine for CO2 capture. CO2 solubility was measured for 1MPZ aqueous solutions at three concentrations - 10 wt%, 30 wt%, and 40 wt% and four temperatures - 313.15K, 343.15K, 373.15K, and 393.15K. The excess enthalpy for 1MPZ + H2O was obtained by the Setaram C80 calorimeter at 303.15 K and 323.15K, within a whole mole-fraction range. The interaction parameters of nonrandom two-liquid model (NRTL) and ENRTL, along with the standard state properties of amine ions - protonated 1MPZ (1MPZH(+), 1MPZH(2+)), 1MPZ carbamate (1MPZCOO(-)), and protonated 1MPZ carbamate (H1MPZCOO) - were regressed from data obtained from this work as well as literature, which agreed with the model calculation. (C) 2015 Elsevier B.V. All rights reserved.