Solid acid catalysts are normally used to catalyze the transesterification of oil with high free fatty acid (FFA) to biodiesel. However, the immiscible phases of methanol-oil-catalyst in the initial reaction mixture usually lead to slow reaction rate and long reaction time. One possible way to overcome this limitation is by using co-solvent that has high solubility in oil and methanol. Therefore, in the present study, the use of biodiesel as co-solvent for transesterification reaction catalyzed by SO(4)(2-)/SnO(2)-SiO(2) (solid acid catalyst) was investigated. It was found that with the use of biodiesel as co-solvent, a high FAME yield of 88.2% (almost 30% higher than without using co-solvent) can be obtained in a shorter reaction time (1.5 h) using the following reaction conditions; reaction temperature of 150 degrees C, methanol to oil ratio of 15 and catalyst loading of 6 wt.% (weight of oil). (C) 2010 Elsevier Ltd. All rights reserved.