The high-pressure phase behavior of the binary system CO2 + 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was determined experimentally by a synthetic method. Bubble points were measured for mixtures of various carbon dioxide concentrations within a temperature and pressure range of (310 to 450) K and (0.5 to 14) MPa, respectively. The results were compared with solubility data obtained by three other research laboratories using different phase equilibria measuring techniques, and good agreement was observed in every case. The effect of the cation alkyl chain length was also investigated by comparing CO2 solubility in four homologous members of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide family in which the alkyl was either ethyl, butyl, hexyl, or octyl. It was shown that CO2 solubility increases almost linearly with increasing alkyl chain length.