Knowledge of the solubility of carbon monoxide in ionic liquids is important for investigating the potential use of ionic liquids in reactions and gas separations. There is very limited information available on the capacity of ionic liquids to capture CO. The solubility of CO in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was investigated experimentally up to temperatures and pressures of 460 K and 10 MPa, respectively. The solubility was shown to be only slightly temperature-dependent, and the P-T diagram for a constant composition mixture is concave-downward with a peak in pressure. By comparing the single-gas solubility results of CO with various other gases in the same ionic liquid, it seems that ionic liquids can potentially be used as gas-separating media. However, although the single-gas solubilities vary considerably among some gases, further studies on mixed (multicomponent) gases should be carried out as the presence of other components can influence the solubility of each gas component. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 3886-3891, 2013