A thermodynamic model was used to estimate enthalpy of solution of carbon dioxide (CO2) in methyldiethanolamine (MDEA) aqueous solutions. The model was based on a set of equations for chemical equilibria, phase equilibria, charge, and mass balances. Non-ideality in the liquid phase was taken into account by interaction parameters fitted to (vapor + liquid) equilibrium data. The enthalpies of solution of CO2 were derived from the model using classical thermodynamic relations and were compared to experimental values obtained in previous works.