"In summary, we have carried out a combined QM calculation and MD simulation study to better understand the removal mechanism of C2H2 from a C2H4 stream by [BMIM][OAc]. Based on the calculated interaction energies, it is proposed that both C2H2 and C2H4 molecules prefer to link to the anion of IL via the hydrogen bonding interaction between an acidic proton of C2H2/C2H4 and the basic oxygen atom in [OAc] − and link to the cation of IL via the C−H···π interaction between the C−H bond of C2H2/C2H4 and the imidazolium ring of the cation. However, the interaction of C2H2 with the cation, anion, and ion pair are found to be stronger than those of C2H4."