This work presents the high-pressure phase behavior of CO2 with six ionic liquids: 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6)], 1-n-octyl-3-methylimidazolium hexafluorophosphate ([C-8-mim][PF6]), 1-n-octyl-3-methylimidazolium tetrafluoroborate ([CB-mim][BF4]), 1-n-butpl-3-methylimidazolium nitrate ([bmim][NO3]), 1-ethyl-3-methylimidazolium ethyl sulfate ([emim][EtSO4]), and N-butylpyridinium tetrafluoroborate ([N-bupy][BF4]). We explored the effect of systematically changing the anionic and cationic components of the ionic liquid on the CO2-ionic liquid phase behavior. For all of the ionic liquids tested, large quantities of CO2 dissolved in the ionic liquid phase, but no appreciable amount of ionic liquid solubilized in the CO2 phase. In addition, the liquid phase volume expansion with the introduction of even large amounts of CO2 is negligible, in dramatic contrast to the large volume expansion observed for neutral organic liquids. Our results seek to elucidate the underlying physical mechanisms of this highly unusual phase behavior.