This paper evaluates and optimizes the production of biodiesel from soybean oil and methanol using sodium hydroxide as catalyst. The study and optimization was carried out at low catalyst concentration (0.2 to 0.6 w/w). The reaction was carried out with application of low-frequency high-intensity ultrasound under atmospheric pressure and ambient temperature in a batch reactor. Response surface methodology (RSM) was used to evaluate the influence of methanol to oil ratio and catalyst concentration on soybean oil conversion into biodiesel. Analysis of the operating conditions by RSM showed that the most important operating condition affecting the reaction was the methanol to oil ratio, while catalyst amount showed little significance in the transesterification reaction. Total consumption of oil was obtained when alcohol to oil ratio of 9:1 and catalyst concentration of 0.2 w/w were applied. (C) 2008 Elsevier B.V. All rights reserved.