Amino acid ionic liquid-based facilitated transport membranes with tetrabutylphosphonium amino acid ionic liquids with glycine, alanine, proline and serine as the anion were prepared and their CO2 permeation properties were evaluated from the physical and physicochemical properties of the amino acid ionic liquids. A tetrabutylphosphonium proline-based facilitated transport membrane showed an excellent CO2 permeability of 14,000 Barrer, twice that of the others investigated, and a CO2/N-2 selectivity of 100 at 373 K under dry conditions. Thermogravimetric measurement showed that the tetrabutylphosphonium proline had a relatively high water-holding ability compared with the other amino acid ionic liquids investigated. The strong water holding ability of tetrabutylphosphonium proline realized the large absorption amount of CO2 and established a large concentration gradient for the CO2-complex across the membrane. The large concentration gradient provided a large driving force for CO2-complex transport through the membrane and increased CO2 permeability. (c) 2012 Elsevier B.V. All rights reserved.