The acidity/basicity of the reaction media has a substantial influence on the efficiency of many reactive processes: therefore, a new class of acidic or basic ionic liquids is gaining special attention due to the possibility of increasing the efficiency of many processes by a wise manipulation of their properties. The absorption of sour gases is one of the processes that can be enhanced by the basic character of the ionic liquid. The fluorination of the cation or anion can also contribute to the gas solubility enhancement. In this work, these two characteristics are evaluated and compared through the study of gas-liquid equilibrium of two ionic liquids, 1-butyl-3-methylimidazolium dicyanamide ([C(4)mim][DCA]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C(4)mim]][Tf2N]), with carbon dioxide (CO2) at temperatures up to 363 K and pressures up to 74 MPa. A thermodynamic model based on the Peng-Robinson equation of state with the Wong-Sandler mixing rule, using the UNIQUAC model for the activity coefficients, was used to describe the experimental data and for the estimation of the Henry's constants. The solubility of CO2 in 1-butyl-3-methylimidazolium dicyanamide is much lower than anticipated on the basis of the reported pK(a) of the anion when compared with the acetate anion. No chemisorption is observed and the solvation enthalpy is quite low, ruling out any Lewis acid/base interaction between the anion and the CO2. The 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid, known to present one of the highest solubilities towards CO2 due to the presence of fluoroalkyl groups, showed a much larger solubility for CO2 than 1-butyl-3-methylimidazolium dicyanamide. (C) 2009 Elsevier B.V. All rights reserved.