The solubilities of caffeine in supercritical CO2, supercritical CO2 + water, supercritical CO2 + ethanol, and supercritical CO2 + water + ethanol were measured with a circulation-type apparatus combined with an on-line Fourier transform infrared (FT-IR) spectrometer at 313.2K and 15.0 MPa. The solubilities of caffeine were determined with the peak absorbances of caffeine at 1190 cm(-1). The solubilities of caffeine increase until water is saturated in supercritical CO2 - The maximum increase rate is 22%. In CO2 +caffeine+ ethanol system, the solubilities of caffeine increase with increasing the concentration of ethanol. The solubility of caffeine becomes five times when 1000 mol m(-3) of ethanol is added. In CO2 + caffeine + water + ethanol system, the solubilities of caffeine are smaller than those with single entrainer of water or ethanol. The shape of the peaks of two C=O stretching bands for caffeine were changed by the, addition of ethanol. It was confirmed that the interaction species of caffeine interacting with ethanol are produced by deconvolution of the C=O stretching bands. The enhancement of solubility for caffeine in supercritical CO2 by the addition of ethanol is due to the hydrogen bonding between caffeine and ethanol. (c) 2006 Elsevier B.V. All rights reserved.