X-ray diffraction measurements for the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], mixed with CO2 were carried out at high pressures using our developed polymer cell. The intermolecular distribution functions obtained for [BMIM][BF4]-CO2 mixtures showed that CO2 molecules are preferentially solvated to the [BF4](-) anion. The similar preferential solvation was previously observed in analogous 1-btuyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6], with a different anion, which is in harmony with the present results in [BMIM][BF4]-CO2. (C) 2010 Elsevier B.V. All rights reserved.