A series of task-specific acidic ionic liquids (TSILs), immiscible with oil, halogen-free and containing -COOH group in the cations, were used for oxidative desulfurization as both the catalyst and extractant. The removal of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) from model diesel at 298 K could reach 96.7% and 95.1%, respectively. The TSIL could be recycled 5 times without any apparent loss of the catalytic activity. Meanwhile, the structures, acidities and interactions between the cation and the anion of TSILs have been investigated by density functional theory (DFT) method, and found that catalytic properties of TSILs are close related to the structures, acidities and extraction capabilities. Furthermore, an oxidative desulfurization mechanism has been proposed. (C) 2010 Elsevier B.V. All rights reserved.