On-line supercritical fluid extraction (SFE) - supercritical fluid chromatography (SFC) / Fourier transform infrared spectrophotometer (FTIR) system has been developed. The solvent polarity parameters of supercritical carbon dioxide were determined as a function of temperature and pressure by this system. The separation behavior of mixtures of lipids by in situ observation with the system was represented by a model introducing the solvent parameters of SC-CO₂ as well as interaction forces of solute with stationary and mobile phases. Furthermore, the consequence of the compressible properties of SC-CO₂ on an enzymatic ester synthesis in the solvent is discussed on the basis of the IR spectroscopic data.