Production of biodiesel in ionic liquids through immobilized Candida antarctica lipase-catalyzed methanolysis of soybean oil was demonstrated. Among tested 23 ionic liquids, the highest fatty acid methyl esters (FAMEs) production after 12h at 50 degrees C was achieved in [Emim][TfO]. The production yield of 80% was eight times higher compared to the conventional solvent-free system. It was around 15% higher than the FAMEs production system using tert-butanol as an additive. The optimum substrate molar ratio of methanol to soybean oil for FAMEs production in [Emim][TfO] was found to be 4:1. Our results of high production yield in ionic liquids show that ionic liquids are potential reaction media for biodiesel production. (c) 2007 Elsevier Inc. All rights reserved.