In this work, the solubility of carbon dioxide (CO2) in seven different ionic liquids (ILs) was measured using a high-pressure sapphire cell, in the pressure range of 8–22 MPa and at two temperatures, 313.2 K and 323.2 K. In order to discuss the influence of the cation, ILs based on bis(trifluoromethylsulfonyl)imide, [NTf2] were chosen. They were coupled with 1-butyl-3-methylimidazolium, [C4mim]+, 1-decyl-3-methylimidazolium, [C10mim]+, 1-butyl-1-methylpyrrolidinium, [Pyrr4,1]+, butyltrimethylammonium, [N4,1,1,1]+, methyltrioctylammonium, [N1,8,8,8]+ and trihexyltetradecylphosphonium, [P6,6,6,14]+cation. The phosphonium based cation was also combined with chloride, [Cl]−, giving an opportunity to discuss the influence of an anion as well. The solubility data obtained in this work were used to evaluate the importance of explicitly including association for describing such systems. For that purpose, we compared the solubility modelling results from two cubic equations of state (Peng–Robinson and Soave–Redlich–Kwong) with those from the Cubic Plus Association Equation of State (CPA EoS) using different association schemes for the ionic liquids. We found that for such systems there is no major advantage in considering ILs as associating components.