The application of on-line Raman spectroscopy for detecting and monitoring a CO2 antisolvent process (GAS) is explored for the first time. Griseofulvin precipitations were carried out in a batch vessel from an acetone liquor, in which the CO2 was introduced at specific rates. Changes of griseofulvin and acetone concentrations upon addition of CO2 were monitored through specific wavenumbers at different pressurization rates (0.10 and 0.45 MPa/min) and courses (continuous or stepwise introduction, final pressure of 10 or 4.8 MPa). The ratio between the solute and solvent signals allowed for accurately detecting the onset of crystallization, for monitoring the depletion of the solution in solute as a function of time, and for estimating the mass of precipitated solids during the crystallization course. Moreover, the Raman spectroscopy provided experimental evidence of the predicted scenario of a crystallization induced by supercritical fluid. (c) 2005 American Institute of Chemical Engineers