Solubility of hydrophilic organic acids in supercritical CO2 (SC-CO2) can be significantly increased utilizing a CO2-soluble Lewis base tri-n-butylphosphate (TBP) as a carrier. Solubilities of the acids carried by TBP are enhanced roughly by one order of magnitude for both citric acid and benzoic acid and a factor of 6 for lactic acid compared with those for each individual acid alone in SC-CO2. Spectroscopic evidence suggests a possible Lewis acid-base complex formation mechanism involving hydrogen bonding between the P=O group of TBP and protons of the acids. With TBP as a carrier, the extraction efficiency of citric acid and lactic acid from aqueous solutions by supercritical CO2 can be enhanced by a factor of 7-10. (c) 2005 Elsevier B.V. All rights reserved.