The sulfur partition coefficient K-N is usually used to characterize the ability of extractive desulfurization (EDS). In this work, several K-N values are measured at 298.15 K, which are between straight-run fuel oil and N-ethylimidazole (EIM), N-methylimidazole (MIM), and its mixture with a dialkylphosphate ionic liquid (IL), viz. N-ethyl-N-methylimidazolium diethylphosphate ([EMIM][DEP]) or N-butyl-N-methylimidazolium dibutylphosphate ([BMIM][DBP]). K-N values between the oil and solvent at varying water content are also measured at 298.15 K. The results indicate that both EIM and MIM have excellent EDS performance with K-N above 3.1 for dibenzothiophene. They have some solubility in fuel oil, but their solubility decreases with addition of IL. The EDS ability of the solvents for a specified sulfur compound followed the order alkylimidazole (EIM > MIM) > mixed solvent (MIM + IL) > IL ([BMIM][DBP] > [EMIM][DEP]); the extractive selectivity of sulfur for a specified solvent followed the order dibenzothiophene (DBT) > benzothiophene (BT) > 3-methylthiophene (3-MT). The used sulfur-containing solvent can be regenerated via a water diluting process followed by a simple distillation process; therefore, the solvent can be reclaimed in a cost-effective way. This work shows that the alkylimidazole solvent and/or its mixture with an IL can be used as a potential extractant for the EDS of fuel oils.