Biodiesel production from soybean oil with methanol was performed in the presence of a Bronsted acidic ionic liquid-based catalyst under ultrasound irradiation. The influences of various parameters on the transesterification reaction, including the amount of catalyst, the molar ratio of methanol to oil, the temperature and the ultrasound power, were investigated. The optimal conditions were: methanol/oil molar ratio of 9:1, 1.0 wt.% catalyst in oil, ultrasound power of 200W, and reaction temperature of 60 degrees C. Under these conditions, the conversion of triglycerides to fatty acid methyl esters was about 93.2% within the reaction time of 60 min. (C) 2012 Elsevier Ltd. All rights reserved.