Amine-functionalized anion-tethered ionic liquids (ILs) trihexyl(tetradecyl) phosphonium asparaginate [P-66614] [Asn], glutaminate [P-66614] [Gln], lysinate [P-66614] [Lys], methioninate [P-66614] [Met], prolinate [P-66614] [Pro], taurinate [P-66614] [Tan], and threoninate [P-66614][Thr] were synthesized and investigated as potential absorbents for CO2 capture from postcombustion flue gas. Their physical properties, including density, viscosity, glass transition temperature, and thermal decomposition temperature were determined. Furthermore, the CO2 absorption isotherms of [P-66614] [Lys], [P-66614] [Tan], [P-66614][Pro] and [P-66614] [Met] were measured using a volumetric method, and the results were modeled with two different Langmuir-type absorption models. The most important result of this study is that the viscosity of [P-66614] [Pro] only increased by a factor of 2 when fully complexed with 1 bar of CO2 at room temperature. This is in stark contrast to the other chemically reacted ILs investigated here and all other amino acid-based ILs reported in the literature, which dramatically increase in viscosity, typically by 2 orders of magnitude, when complexed with CO2. The unique behavior of [P-66614] [Pro] is likely due to its ring structure, which limits the number and availability of hydrogen atoms that can participate in a hydrogen bonding network. We found that water can be used to further reduce the viscosity of the CO2-complexed IL, while only slightly decreasing the CO2 capacity. Finally, from temperature-dependent isotherms, we estimate a heat of absorption of -63 kJ/mol cif CO2 for the 1:1 reaction of CO2 with [P-66614] [Pro], when we use the two-reaction model.