For the halide-free imidazolium-based ionic liquids with a methanesulfonate anion, the solubility results of carbon dioxide (CO2) are presented at pressures up to approximately 45 MPa and temperatures ranging from 303.15 K to 343.15 K. The solubility was determined by measuring bubble pressures of mixtures of CO2 and ionic liquid using a high-pressure equilibrium apparatus equipped with a variable-volume view cell. The ionic liquids studied in this work were 1,3-dibutylimidazolium methanesulfonate, 1-ethyl-3-methylimidazolium methanesulfonate, 1-butyl-3-methylimidazolium methanesulfonate, and 1,3-dimethylimidazolium methanesulfonate. A sharp increase of equilibrium pressure was observed at high CO2 compositions. In ionic liquids, the CO2 solubility was elevated according to the increase of the total length of alkyl chains linked to the imidazole of the ionic liquids, and this increase became more apparent at higher pressures. The ionic liquids with a methanesulfonate anion gave a lower CO2 solubility than the ionic liquids with a fluorinated substituent such a fluoroalkyl group on their anions.