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It is important to understand the mechanism of toluene absorption by phosphonium ionic liquids (PILs) for volatile organic compound removal with ionic liquids. In this work, the molecular structures of trimethyl(butyl)phosphonium tetrafluoroborate ([P 1114 [BF 4), trimethyl(butyl)phosphonium bis(trifluoromethylsulfonyl)imide ([P 1114 [Tf 2 N]), tributyl(propyl)phosphonium tetrafluoroborate ([P 4443 [BF 4) and tributyl(propyl)phosphonium bis(trifluoromethylsulfonyl)imide ([P 4443 [Tf 2 N]) were optimized by quantum chemical calculations at the GGA/PW91 level with the DNP basis set of density functional theory (DFT). The most stable configurations for toluene absorption by these PILs with multiple sites were also obtained by calculation under the same conditions. The main parameters for the most stable structure, data for the Mulliken charge and frontier molecular orbitals were further determined and discussed. It was found that the interactions of PILs with toluene are mainly composed of hydrogen bonds, C-H···π bond interactions and electrostatic attraction, and most of the calculated frontier molecular orbitals are concentrated on the toluene molecule. Additionally, analysis showed that [BF 4 − is inclined to form hydrogen bonds with cations and the methyl groups of toluene, and [Tf 2 N]− tends to attract toluene via electrostatic attraction. Investigation of the absorption energy with multiple sites indicated that PILs have great performance for toluene absorption. Analysis of the ionic size and geometric structure of the PILs demonstrated that the space available for accommodating toluene is an important condition for toluene absorption and is mainly provided by cations. In particular, PILs with cations having suitable long alkyl side chains show excellent capacity for toluene absorption. Moreover, NBO and AIM analyses confirmed the existence of weak closed-shell interactions (hydrogen bonds and C-H···π bonds) between toluene and PILs. The experimental results obtained for the static toluene absorption by the PILs provide a great supplement and confirmation of the calculations [Display omitted] • Structure of phosphonium cation is conducive for multiple sites absorption. • Space for accommodating toluene is mainly provided by phosphonium cation. • Suitable long alkyl chain is necessary for the high toluene absorption. • Static toluene absorption amount for [P 8883 [BF 4 is as high as 2840 mg/g. [ABSTRACT FROM AUTHOR]