The interaction between ethanethiol molecule and either anhydrous FeIII chloride anions or 1-butyl-3-methylimidazolium (BMIM+) cations of ionic liquids (ILs) was investigated using Density Functional Theory approach and results were correlated with our previous experimental results on extractive desulfurization (EDS), where ILs containing anionic FeIII species show excellent performance to remove sulfur compounds from natural gasoline, especially when there exists an excess of FeCl3 in the BMIM+FeCl4− IL since this mixture contains binuclear anions Fe2Cl7−, whose Fe―Cl―Fe bonds are larger and less strong than Fe―Cl bonds of mononuclear anions FeCl4−, being then the former bonds activated for ethanethiol chemisorption. Molecular frontier orbitals and Mulliken atomic charges reveal that the high desulfurization performance could be due to a Dewar–Chatt–Duncanson-like mechanism of electron donation–backdonation among ethanethiol sulfur and transition metal centers of Fe2Cl7− anions, and this mechanism is promoted because of the symmetry affinity among the ethanethiol HOMO and the atomic orbital t2g on Fe sites in Fe2Cl7− LUMO. On the other hand, when there is no excess of FeCl3, EDS increases with the size of N-alkyl substitutes in BMIM+ due to ethanethiol physisorption by these cations. Display omitted ► DFT study of desulfurization of ionic liquids containing FeIII. ► Interaction between ionic liquids containing Fe and ethanethiol was investigated. ► Dewar–Chatt–Duncanson-like mechanism can explain our experimental results.