Abstract: AIMTo establish a suitable condition for extraction of phenylalanine (Phe), 5-hydroxytryptophan (5-OH-Trp) and four diastereomeric salts, (1R,2S)-ephedrine-(2S,3S)-tartaric acid, (1R,2S)-ephedrine-(2R,3R)-tartaric acid, (1S,2S)-pseudoephedrine-(2S,3S)-tartaric acid, (1S,2S)-pseudoephedrine-(2R,3R)-tartaric acid in supercritical fluid extraction and to assess the solubilities of Phe, 5-OH-Trp and the four diastereomeric salts in CO₂. METHODSSingle-pass method and HPCE. RESULTSThe solubilities of Phe, 5-OH-Trp and the four diastereomeric salts in CO₂ were determined over temperature and pressure ranges of 25～50℃ and 6.32～34.03 MPa respectively. The experimental results showed that the solubilities of Phe, 5-OH-Trp and the four diastereomeric salts do not increase with density of CO₂. There existed a maximum in the critical region of CO₂. CONCLUSIONThe dramatically high solubilities in the pressure of 6.32～7.78 MPa show a critical behavior, which can be explained by critical characteristic through thermodynamics analysis. The results suggest that the separation of Phe, 5-OH-Trp and the four diastereomeric salts is more efficient in critical region of CO₂.