FT-IR spectroscopy was used to measure the liquid phase speciation in water-diethanolamine (DEA)-CO2 system, the CO2 loading varying from 0.04 to 0.9. The DEA mass fractions in the unloaded solution were 0.2, 0.3 and 0.4, and the temperature varied from 298.1 K to 333.1 K. The concentrations of carbamate and molecular CO2 have been experimentally determined and the other species concentrations (DEA, protonated DEA, bicarbonate) have been deduced from mass balances and electroneutrality relation. These values, combined with the measurement of CO2 partial pressure in equilibrium with the liquid phase, allow the determination of apparent CO2 Henry constant (H-CO2)(DEAsolution) original values. These results show that (H-CO2)(DEAsolution) is independent of the CO2 loading. Moreover, it is shown that the increase of temperature results in a decrease of (H-CO2)(DEAsolution). (c) 2012 Elsevier B.V. All rights reserved.