A transesterification reaction of esters from soybean oil with ethanol assisted by low-frequency ultrasound (20 kHz) was performed in the presence of a potassium hydroxide catalyst. The effects of the following variables on the reaction performance were studied simultaneously through a full two-level factorial design 2(3): the concentration of the catalyst, the oil:ethanol molar ratio and the sonication time. The three investigated factors significantly affected the yield. Further experimentation was performed using a three-step reaction. An ethyl ester yield of approximately 98% was attained in the third step. The physicochemical properties of the products were compared with the technical limits of biodiesel, and the results suggest that the ultrasound-assisted ethanol transesterification reaction could be a viable and efficient method for the production of quality biodiesel from soybean oil. (C) 2011 Elsevier B.V. All rights reserved.