In this study, solubility of Acid Red 57 (AR57), which is an anionic dye, in supercritical CO2 was investigated using an ion-pairing with dodecyltrimethylammonium (DTMA) bromide. The measurements of AR57 and AR57-DTMA in supercritical CO2 without/with methanol as a modifier solvent were carried out at temperatures from 45 to 100 degrees C and pressures from 250 to 325 bar. Although AR57 is insoluble in supercritical CO, even with methanol as a modifier solvent, AR57-DTMA can dissolve in methanol modified supercritical CO2. The hydrophobic. ion-pairing provides a possibility for the solubility of hydrophilic dye in supercritical CO2. The solubilities of AR57-DTMA were measured in methanol modified supercritical CO, and the results were correlated by using a semi-empirical equation. (c) 2006 Elsevier B.V. All rights reserved.