We evaluated the absorption characteristics and phase state of a biphasic CO₂ absorbent using 2-amino-2-methyl-1-propanol (AMP), diethylene glycol diethyl ether (DEGDEE), and water (H₂O). We set the AMP concentration at 30 wt%, and adjusted the H₂O and DEGDEE weight fractions. It was experimentally confirmed that phase separation of the absorbents occurred due to the progress of CO₂ absorption. As the concentration of water decreased, it was separated into a lower phase, which consisted almost entirely of AMP, H₂O and absorbed CO₂, and an upper phase, which consisted of mostly DEGDEE. In the experiment, an absorbent composition of 30 : 20 : 50 wt.% (AMP:DEGDEE : H₂O) was maintained in the liquid state at 313 K and a CO₂ partial pressure of 0.1 atm in the absorption step and at ≥ 343 K and a CO₂ partial pressure of 1.0 atm in the regeneration step. Under CO₂ capture conditions, this adsorbent was separated into two-liquid states in the absorption step, whereas it exhibited a uniform single phase in the regeneration step. Therefore, it could operate under CO₂ capture conditions as a biphasic absorbent. The amount of CO₂ cyclic absorption of this absorbent was twice that of conventional monoethanolamine (MEA) absorbents when the regeneration temperature was set at 363 K.