The vapor-liquid equilibrium of the binary system CO2+1-butyl-3-methylimidazolium nonafluorobutyl sulfonate ([BMIM][NfO]) was measured over a temperature range of 298.2-323.2 K at intervals of 5.0 K for CO2 mole fraction ranging from 0.137 to 0.900 using a high-pressure variable-volume view cell. The Peng-Robinson equation of state was then applied with two-parameter mixing rules over the same range and the results compared with the experimentally obtained data. Increasing the alkyl chain length in perfluorinated sulfonate from methyl to butyl markedly increased the CO2 solubility. To investigate the effect of the number of fluorine atoms in the anion on the phase behavior of imidazolium based ionic liquid, these experimental results were then compared with those reported in previous experimental studies of 1-alkyl-3-methylimidazolium cations and with modeling data. It looks likely that both the number of fluorine atoms in the anion and the presence of S=O groups play an important role in designing CO2-philic molecules.