Solubilities of glycoside isoflavones (daidzin, genistin and glycitin) and their aglycons (daidzein, genistein and glycitein) in supercritical carbon dioxide (SCCO(2)) were measured. In the SCCO(2)-only system, the solubility of glycoside isoflavones was very low. Adding over 0.1 molar fraction of ethanol to SCCO(2) increased the solubility of glycoside isoflavones. The solubility was almost proportional to the third power of the molar fraction of ethanol. Adding ethanol significantly affected the solubility of glycoside isoflavones. In particular, the solubility of daidzin was remarkably increased by ethanol addition. The solubility of six tested isoflavones in SCCO(2) with ethanol was correlated with the second virial coefficient. The solubility depended on seventh power of the molar fraction of ethanol in SCCO(2). This suggested that the addition of ethanol facilitated cluster formation of CO(2) molecules in the supercritical phase, enhancing the solubility of isoflavones. A SCCO(2) and ethanol binary system was especially effective in extracting glycoside isoflavones. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.