The solubilities of ionic liquids (ILs) 1-butyl-3- methylimidazolium hexafluorophosphate ([bmim] [PF6]) and 1-butyl-3- methylimidazolium tetrafluoroborate ([bmim] [BF4]) in supercritical (sc) CO2 with and without organic cosolvents were determined at 40.0 degreesC and 15.0 MPa. The mole fraction of a cosolvent was up to 0.3. The solubilities of the ILs in sc CO2 + polar cosolvent mixtures increased dramatically as the cosolvent concentration exceeded 10 mol %, but the solubility was extremely low when the concentration of the cosolvent was lower than 10 mol %. The effect of a cosolvent on the solubility of the ILs in sc CO2 depends mainly on its polarity. The stronger the polarity of a cosolvent, the larger the effect. The ability of the mixed cosolvent to enhance the solubility is intermediate between the two pure cosolvents.