Phase-change ionic liquids, or PCILs, are salts that are solids at normal flue gas processing temperatures (e.g., 40-80 C) and that react stoichiometrically and reversibly with CO2 (one mole of CO2 for every mole of salt at typical postcombustion flue gas conditions) to form a liquid. Thus, the melting point of the PCIL-CO2 complex is below that of the pure PCIL. A new concept for CO2 separation technology that uses this key property of PCILs offers the potential to significantly reduce parasitic energy losses incurred from postcombustion CO2 capture by utilizing the heat of fusion (Delta H-fus) to provide part of the heat needed to release CO2 from the absorbent. In addition, the phase transition yields almost a step-change absorption isotherm, so only a small pressure or temperature swing is required between the absorber and the stripper. Utilizing aprotic heterocyclic anions (AHAs), the enthalpy of reaction with CO2 can be readily tuned, and the physical properties, such as melting point, can be adjusted by modifying the alkyl chain length of the tetra-alkylphosphonium cation. Here, we present data for four tetrabutylphosphonium salts that exhibit PCIL behavior, as well as detailed measurements of the CO2 solubility, physical properties, phase transition behavior, and water uptake for tetraethylphosphonium benzimidazolide ([P-2222] [BnIm]). The process based on [P-2222] [BnIm] has the potential to reduce the amount of energy required for the CO2 capture process substantially compared to the current technology that employs aqueous monoethanolamine (MEA) solvents.