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Progress in the transesterification of vegetable oils has been achieved using ionic liquids. Ionic liquids have been found to provide an ideal medium for increasing the biodiesel yield, stabilizing the inorganic acid and removing the glycerol by-product. In this present study, transesterification of triglycerides common in activated sludge oil with methanol to biodiesel was investigated using various imidazolium-based ionic liquids under acidic conditions. The influences of the nature of the anion and the alkyl groups on the imidazolium cation, ionic liquid dosage, reaction temperature and reaction time were also explored.