Accurate modeling of thermodynamic properties for CO2 absorption in aqueous alkanolamine solutions is essential for the simulation and design of such CO2 capture processes. In this study, we use the Electrolyte Nonrandom Two-Liquid activity coefficient model to develop a rigorous and thermodynamically consistent representation for the MDEA−H2O−CO2 system. The vapor−liquid equilibrium (VLE), heat capacity, and excess enthalpy data for the binary aqueous amine system are used to determine the NRTL interaction parameters for the MDEA−H2O binary. The VLE, heat of absorption, heat capacity, and NMR spectroscopic data for the MDEA−H2O−CO2 ternary system are used to identify the NRTL interaction parameters for the molecule−electrolyte binaries and the previously unavailable standard-state properties of the amine ion, MDEA protonate. The calculated VLE, heat of absorption, heat capacity, and the species concentrations for the MDEA−H2O−CO2 system are compared favorably to experimental data.