This paper presents the high-pressure phase behavior of the binary system 1-octyl-3-methylimidazolium tetrafluoroborate ([omim][BF4])+carbon dioxide (M) in the mole fraction range 0.1 < x(CO2) < 0.75, and in the pressure and temperature range of 0.1 - 100 MPa and 303 - 363 K, respectively. High solubilities of CO, for x(CO2) < 0.6 were established at relatively low-pressure, while for x(CO2) > 0.6 a drastic increase of the equilibrium pressure was observed. From the experimental work, it could be concluded that, according to the Scott and Van Konynenburg classification of fluid phase equilibria, the system [omim] [BF4] + CO2 most likely shows type-III fluid phase behavior. (c) 2006 Elsevier B.V. All rights reserved.