The phosphonium- and ammonium-based ionic liquids combined with three anions, such as chloride, tetrafluoroborate and trifluoromethanesulfonate were used to perform a systematic study by theoretical method. The results demonstrate that the cation-anion interaction energies between phosphonium-based ionic liquids are slightly larger than that of their analogues. The larger phosphonium cations can form associated species that chloride, tetrafluoroborate, and trifluoromethanesulfonate anions are able to approach the centre of positive charge more closely than it can in the equivalent nitrogen-based cation due to the less steric effect of phosphorium cation. (C) 2014 Elsevier B.V. All rights reserved.