Extractive and oxidative desulfurization of fuel oil using ionic liquids (ILs) as solvents have been intensively studied recently. In such processes, the mutual solubility of ILs and fuel oil is still a major concern. Less mutual solubility is desired to reduce the loss of ILs and the contamination of fuel oil. To screen the ideal ILs with less mutual solubility with fuel oil and to understand the effects of ILs structural characteristics on the mutual solubility, we employed Conductor-like Screening Model for Real Solvents (COSMO-RS) to calculate the mutual solubility of 1830 ILs and model fuel oil. The influences of ILs structural characteristics such as cationic nature, cationic alkyl chain length, cationic symmetry, anionic nature, anionic alkyl chain length and functional group on the mutual solubility are investigated from microlevel view with sigma-profile, sigma-moment and COSMO-RS energies. The mutual solubility is strongly dependent on cation or anion species and is highly influenced by van der Waals (vdW) and hydrogen-bonding (HB) energies. Cations with smaller non-polarity, shorter alkyl chain length and less symmetry tend to have weaker vdW energies and smaller mutual solubility, while anions with larger polarity tend to have stronger HB energies and smaller mutual solubility. The functional groups also show remarkable effects on mutual solubility; those functional groups that decrease the non-polarity and vdW energies or increase the polarity and HB energies favor the small mutual solubility. Moreover, experimental determinations of the mutual solubility indicate [C(1)pyr]H2PO4 is a good solvent for desulfurization. This work provides the theoretical basis to design and select the ILs, which have small mutual solubility with fuel oil. (C) 2016 Elsevier Ltd. All rights reserved.