A flow mixing unit using a SETARAM C-80 calorimeter, developed for measuring the enthalpy of solution of two fluids, has been used to measure enthalpies of absorption of carbon dioxide in a 30 wt.% aqueous solution of methyldiethanolamine (MDEA) at three temperatures 313.15, 353.15, 393.15 K and three pressures 2.0, 5.0, 10.0 MPa. We have established that the effect measured by calorimetry corresponds not only to the absorption of CO2 in the aqueous solution but also to the vaporisation of water into the carbon dioxide depending on the temperature and the pressure of the experiment. The enthalpies measured by calorimetry were compared with those calculated from solubility measurements and a reasonable agreement within the accuracy of measurement and calculation was found.