Amino acid ionic liquid trihexy(tetradecyl)phosphonium methioninate [P-66614][Met] and prolinate [P-66614][Pro] absorb CO2 in nearly 1:1 stoichiometry, surpassing by up to a factor of 2 the CO2 capture efficiency of previously reported ionic liquid and aqueous amine absorbants for CO2. Room temperature isotherms are obtained by barometric measurements in an accurately calibrated stirred cell, and the product identity is confirmed using in situ IR. Density functional theory (DFT) calculations support the 1:1 reaction stoichiometry and predict reaction enthalpies in good agreement with calorimetric measurements and isotherms.