Treatment and separation of multicomponent gases using absorption/desorption cycles in aqueous solutions is a very well-known and efficient method, used in natural gas treatment, biogas purification, greenhouse gas control … More specifically, the use of aqueous solutions of amine is used with benefit for CO 2 removal in mixed gases. However, the energetic cost of this method is highly expensive in carbon capture processes. A new class of amine is considered to decrease the cost of the regeneration: the demixing amines. These amines present a lower critical solution temperature that can be used with benefit in post-combustion processes. The LCST depends strongly on the chemical structure of the amine and on the presence and quantity of dissolved gas. The aim of this chapter is to evaluate the influence of physical absorbent, namely triethylengycol, on the liquid-liquid equilibria, and on the associated thermodynamic properties (vapor-liquid equilibria, heat capacities, densities, heat of solutions) of the solutions needed for design of the carbon capture process. X.1 Introduction Chemical absorption of acid gases by amine based solvents has found applications in a wide variety of industries including gas processing and the removal of CO 2 from synthesis gas in the production of hydrogen or ammonia. Other applications of this technology is the purification of biogas [1] or CO 2 removal from post-combustion gases in power plants [2].