Abstract The acetylacetonate complexes of oxovanadium(IV): VO(X-acac)2 (X=Cl, CH3 and CH3CH2) were structurally characterized in several alkylimidazolium ionic liquids by visible absorption and EPR spectroscopies. VO(Me-acac)2 and VO(Et-acac)2 showed solvatochromism in the selected ionic liquids (a λ1 shift of ca. 120nm was observed) and most complexes showed two species in the anisotropic EPR spectra in several ionic liquids. It was shown that the ionic liquids anions have coordinating ability and the basicity order found was: NTf2−<OTf−<BF4−, in agreement with previous findings. An extraction and catalytic oxidation desulfurization (ECODS) system composed of VO(X-acac)2, H2O2 and ionic liquids was tested in the removal of benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) from a model oil (n-octane) at ambient temperature. The influence of the catalyst, ionic liquid, substrate and the molar ratio of the oxidant (H2O2) and the S-containing molecule were examined. The reaction rates of the oxidative desulfurization reaction were found to increase with the molar ratio of H2O2 and the S-containing molecule, as expected, however a ratio of 4 was enough to achieve >95% conversion to sulfone. The ionic liquid that showed the best performance with all substrates was bmimNTf2. In bmimOTf and bmimBF4 the activity of all catalysts in the ECODS system was low to moderate, which is probably due to the higher coordinating ability of these anions.