This　study　systematically　investigated　the　effect　of　anionic　structure　on　toluene　absorption　with　imidazolium-based　ionic　liquids　(ILs).　The　thermodynamic　model　COSMO-RS　was　used　to　screen　three　ILs　(i.e.,　[EMIM]　[De],　[EMIM][Pe],　and　[EMIM][Ca])　with　carboxyl-based　anions　as　candidates,　from　a　database　of　285　ILs.　The　absorption　capacity　of　toluene　in　ILs　was　measured　experimentally.　The　results　showed　that　[EMIM][De]　had　the　highest　toluene　absorption　capacity　(173.3　mg/g)　among　all　the　ILs.　The　molecular　mechanism　of　toluene　absorption　with　different　ILs　was　revealed　through　theoretical　calculations.　It　was　found　that　the　Van　der　Waals　(vdW)　and　C-H　center　dot　center　dot　center　dot　pi　interactions　jointly　dominate　the　[De](-)-/[Pe](-)-/[Ca](-)-toluene　pairs,　while　only　the　vdW　interaction　dominates　the　[Tf2N](-)-toluene　pair.　Furthermore,　[EMIM][De]　has　the　largest　free　volume　among　the　four　ILs.　These　findings　and　conclusions　can　be　directly　applied　to　task-specific　IL　screening　and　design　for　absorbing　other　aromatic　VOCs　due　to　the　similar　absorption　mechanism.