dThe phase behavior of a binary mixture consisting of a supercritical fluid and an imidazolium-based ionic liquid was studied experimentally. Carbon dioxide (CO2) and 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim] [BF4]) were the selected supercritical fluid and ionic liquid (IL), respectively. A synthetic method was used to measure the phase behavior of the binary system CO2 + [hmim] [BF4]. Bubble-point pressures of this binary mixture were reported for CO2 concentrations ranging from (10.3 up to 70.3) mol % and within a temperature range of (293.18 to 368.16) K. The experimental results obtained in this work were compared with the available phase behavior data of the binary system CO2 + 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim] [PF6]) to investigate the effect of the anion group on the phase behavior of this type of system. The three-phase liquid-liquid-vapor (LLV) equilibrium of this system was also measured. On the basis of the LLV measurements, the type of fluid phase behavior of this system most likely will be type III according to the classification of Scott and Van Konynenburg. The CO2 + [hmim] [BF4] binary system has a two-phase liquid-vapor region extending up to very high pressures.