The enthalpies of solution (Delta H-sol) of carbon dioxide (CO2) in two aqueous solutions (w = 0.1500 and w = 0.3000) of diethanolamine (DEA) have been measured at two temperatures ((322.5 and 372.9) K) and pressures up to 3 MPa. Measurements were carried out by a flow calorimetric technique using a custom-made flow-mixing unit combined with a SETARAM C-80 isothermal differential heat-flux calorimeter. Enthalpies of solution of CO2 (Delta H-sol) have been obtained as function of loading, alpha (moles CO2/mol amine). Influences of temperature, pressure, and absorbent composition have been discussed. Solubility data of the gas into the different absorbent (s) were derived from the enthalpic data. The experimental enthalpies of solution (Delta H-sol) of carbon dioxide (CO2) in the two aqueous solutions of diethanolamine have been compared with data derived from a rigorous thermodynamic model of phase equilibria based on a gamma-phi approach. Interaction parameters were chosen to be adjustable parameters in this model and were fitted to vapor-liquid equilibrium data. Several formulations for the amine protonation and carbamate formation equilibrium constants have been tested. The different contributions to the enthalpy of solution of CO2 in aqueous solutions of DEA have been analyzed.