A high-pressure variable-volume view cell was used to measure the phase equilibria of a 1-hexyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([HMMIM][Tf2N]) and CO2 binary system in the temperature range of (298.2 to 323.2) K at intervals of 5.0 K. In addition, the vapor-liquid equilibrium of a 1-octyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([OMMIM][Tf2N]) and CO2 binary system was determined in the temperature range (288.2 to 323.2) K at 5.0 K intervals. The Peng-Robinson equation of state with the van der Waals two-parameter mixing rule was used to correlate the vapor-liquid equilibria of CO2 and ionic liquids. The effect of the alkyl chain length in the cation on the phase behavior of a trisubstituted-imidazolium-cation-based ionic liquid and CO2 binary system was investigated.