NOVELTY - Low-volatility aqueous composition comprises thermally stable amine, which is at 3-20 mols/kg of water loaded with carbon dioxide, which is at 0.1-0.6 mols/thermally stable amine equivalent.    USE - The low-volatility aqueous composition is used for removing acid gas from gas mixture or gaseous stream, which is flue gas, natural gas, hydrogen gas, and/or synthesis gas. The acidic gas is hydrogen sulfide (H2S), carbonyl sulfide, CS2, mercaptans, and/or preferably carbon dioxide. (all claimed)    ADVANTAGE - Concentrated aqueous thermally stable amines are more stable to oxidative and/or thermal degradation as compared to conventional solutions, such as monoethanolamine (MEA). Solutions of thermally stable amine loaded with carbon dioxide do not degrade even at temperatures as high as 150 degrees C, while MEA undergo degradation (up to 50%) at 120 degrees C. Thus, the low-volatility aqueous composition can be used at higher pressures and/or temperatures (i.e. less than 175 degrees C). The ability of the aqueous composition to operate at higher pressures and/or temperatures reduces the amount of energy needed to perform the removal of acidic gas from gas mixture, where reduction of the amount of energy is 10-30%. Solutions comprising thermally stable amine absorb carbon dioxide at faster rates. The use of solutions comprising thermally stable amine results in increased in carbon dioxide absorption rates ranging from 20 to 100%. Such increased carbon dioxide absorption rates enable absorber configurations, which require less packing and pressure drop. The thermally stable amine may be recovered following absorption of carbon dioxide.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) reduction of thermally stable amine volatility comprising providing a solvent that comprises thermally stable amine; and adding carbon dioxide to the solvent;    (2) increasing the solubility of solid thermally stable amine comprising providing a thermally stable amine to a solution of 0.1-0.6 mol carbon dioxide per amine equivalent;    (3) a method comprising contacting a gas mixture or gaseous stream having an acidic gas with a solvent, which comprises thermally stable amine; and allowing the acid gas to transfer from the gaseous stream to the solvent; and    (4) a method comprising contacting gaseous stream with a solvent comprising thermally stable amine, in an absorber, where gaseous stream comprises an acidic gas; allowing the acid gas to transfer from the gaseous stream to the solvent; forming a purified gaseous stream and a rich solvent stream; and routing the rich solvent stream through a multistage flash stripper or a multistage stripper, where multistage flash stripper is operated at less than 175 degrees C.