The effect of the cation and anion on the liquid-liquid equilibrium of aprotic heterocyclic anion (AHA) ionic liquids (ILs) and water was studied by volumetric Karl Fischer titration. It was found that the cation size and the anion nature impact the hydrophobicity of the ionic liquid.In the context of post combustion CO2 capture, small tetraalkylphosphonium cations and different AHA anions were probed for CO2 capture. The AHA ILs studied showed high affinity for CO2 binding; however, some of them showed low chemical stability and high viscosity even when paired with smaller alkyl chain cations. As well, a proton NMR spectroscopy method was developed, in order to obtain a full understanding of the reaction mechanism, kinetics and equilibrium values of the IL-CO2-H2O reaction. The results indicate that the AHA anions undergo reprotonation, although the reaction appears to be reversible. The rate, equilibrium values, and CO2 solubility vary significantly with the nature of the anion.