Carbon Capture and Storage (CCS) is a concrete option for CO 2 mitigation in the atmosphere. One option is the CO 2 capture from post-combustion industrial effluents followed by storage in secured sites. Capture processes can be based on selective absorption/desorption cycles of gas into aqueous solutions of alkanolamines[1]. The cost of CO 2 treatment with classical alkanolamines is a limitation for the intensive use of this technology. The SIMODEX project is dealing with coupling experimental developments, molecular simulation and thermodynamic modeling to understand structureproperties relationships, and to develop transferable conclusions to optimize processes at low costs. This chapter will focus on the use of ATR-IR (Attenuated Total Reflection-infrared) spectroscopy to quantify the speciation in mixtures containing alkanolamine and CO 2 , in absence of water, in order to validate the thermodynamic models already used, and to give crucial information for molecular simulation developments.