A series of substituted benzoate-based ionic liquids (SBILs) were synthesized to investigate the effect of alkalinity of ionic liquids on the selective absorption of acidic gases. The alkalinity of SBILs can be easily tuned through altering the substitutions on the benzene ring. The performance of SBILs for selective absorption of acidic gases (e.g., solubility of SO2, H2S and CO2, selectivity of SO2/CO2 and H2S/CO2) was systematically investigated and found to be highly dependent on the alkalinity of SBILs. Some thermodynamic properties (e.g., Henry's constants and reaction equilibrium constants) were also calculated and correlated with the alkalinity of SBILs. The optimized alkalinity for ionic liquids with maximized selectivity of SO2/CO2 or H2S/CO2 was thus figured out. Furthermore, to achieve a compromise between capacity and selectivity, upper bounds were proposed as criteria for evaluating whether ionic liquids are with both high capacity and selectivity. The results obtained in this work provide interesting clues for the future design of ionic liquids based on the insights into the effect of alkalinity. (C) 2016 Elsevier B.V. All rights reserved.