Carbon dioxide solubility {(vapor + liquid) equilibria: VLE} in ionic liquid, 1-butyl-3-methylimidazolium acetate ([bmim][Ac]), has been measured with a gravimetric microbalance at four isotherms about (283, 298, 323, and 348) K up to about 2 MPa. (Vapor + liquid + liquid) equilibria (VLLE: or liquid-liquid separations) have also been investigated with a volumetric method used in our previous works, since the present analysis of the VLE data using our equation-of-state model has predicted the VLLE at CO2-rich side solutions. The prediction for the VLLE has been confirmed experimentally. CO2 solubilities at the ionic liquid-rich side show extremely unusual behaviors; CO2 dissolves in the ionic liquid to a great degree, but there is hardly any vapor pressure above these mixtures up to about 20 mol% of CO2. It indicates that CO2 may have formed a non-volatile or very low vapor pressure molecular complex with the ionic liquid. The thermodynamic excess properties (enthalpy, entropy, and Gibbs free energy) of the present system do support such a complex formation. We have conducted several other experiments to investigate the complex formation (or chemical reactions), and conclude that a minor chemical reaction occurs but the complex formation is reversible without much degradation of the ionic liquid. (C) 2007 Elsevier Ltd. All rights reserved.