Efficient and recyclable liquid-liquid extraction and catalytic oxidative desulfurization (ECODS) systems for the removal of refractory sulfur compounds from liquid fuels are reported that use the cyclopentadienyl molybdenum tricarbonyl complexes [CpMo(CO)(3)Me] (1), [CpMo(CO)(3)(CH2-PC6H4-CO2Me] (2) and [CpMo(CO)(3)CH2COOH] (3) as catalyst precursors. An ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, was used as both extractant and reaction medium, entrapping the active homogeneous Mo-VI catalysts that are formed in situ under the operating catalytic conditions (aqueous H2O2 as oxidant, 50 degrees C). The high sulfoxidation activity of the catalyst formed from 1 was largely responsible for enabling > 99% desulfurization within 1 h of a model oil containing 1-benzothiophene, dibenzothiophene, 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene (2000 ppm S). The IL/catalyst phase could be repeatedly recycled with no loss of desulfurization efficiency. By sequentially performing extractive desulfurization and ECODS steps, 83-84% sulfur removal was achieved for untreated real diesel and jet fuel samples with initial sulfur contents of ca. 2300 and 1100 ppm, respectively.