Ionic liquids are environmentally benign solvents because of their less volatile and nonflammable natures. Ionic liquids have very high and selective solubilities of acidic gases such as CO₂, SOx, and NOx. By means of X-ray diffraction measurements, we have revealed that CO₂ dissolved in an ionic liquid is preferentially solvated to anion species. This implies that there is the Lewis acid-base interaction between the positive carbon atoms of CO₂ and negative fluorine atoms of anion in the ionic liquids. In contrast, for lack of specific interaction site, nonpolar gases such as H₂ and N₂ do not generally dissolve in ionic liquids. The unique physical absorption property in ionic liquids should open doors for new gas separation methods.