CO₂ removal (or separation) is the key step for biogas upgrading. This research aims to investigate aqueous solutions of amino acid ionic liquids to achieve effective CO₂ separation. In this work, three cholinium-based amino acid ionic liquids ([Cho][AA]s) (i.e. cholinium glycinate ([Cho][Gly]), cholinium alaninate ([Cho][Ala]) and cholinium prolinate ([Cho][Pro])) were synthesized and characterized. The effect of water on the viscosity, CO₂ absorption loading (m and α) and apparent absorption rate constant was systematically studied. The CO₂ absorption mechanism in the aqueous solution of [Cho][Gly] was explored by ¹³C Nuclear Magnetic Resonance (NMR). The results demonstrate that the absorption loading (m) and viscosity increase with increasing IL concentration, while the apparent absorption rate constant decreases. The absorption loading decreased with increasing temperature. The CO₂ absorption mechanism in the aqueous [Cho][Gly] solution started with the chemical reaction to form carbamate at low absorption loading (α), and followed by the hydrolysis of carbamate and CO₂ hydration reaction at high absorption loading (α). Moreover, the aqueous solution with 5 wt % [Cho][Gly] showed the highest regeneration efficiency, and the absorption and regeneration performance of the aqueous solution of [Cho][Gly] was compared with commercial CO₂ absorbents with promising results.