To study the potential of performing hydrogenation reactions in ionic liquid-CO2 media, the high-pressure phase equilibrium of the ternary system 1-butyl-3-methylimidazolium tetrafluoroborate (bmim[BF4]) + 4-isobutylacetophenone + carbon dioxide (CO2) has been investigated. Experiments were performed in the Cailletet apparatus in a temperature range from 278.15 up to 368.15 K while pressures up to 14 MPa were applied. In this system, not only liquid-vapor boundaries were found, but also at high CO2 concentrations liquid-liquid biphasic equilibrium occurred.(1) A comparison between the experimental results of this work, the data collected for the binary system bmim[BF4] + CO2 and a ternary system with acetophenone as the solute shows that the type of solute added can dramatically change the phase behavior of the system. (C) 2007 Elsevier B.V. All rights reserved.