BACKGROUND: In this work we describe the synthesis of ethyl esters, commonly known as biodiesel, using refined soybean oil and ethanol in a solvent-free system catalyzed by lipase from Thermomyces lanuginosus. Central composite design and response surface methodology (RSM) were employed to optimize the biodiesel synthesis parameters, which were: reaction time, temperature, substrate molar ratio, enzyme content, and added water, measured as percentage of yield conversion. RESULTS: The optimal conditions obtained were: temperature, 31.5 degrees C; reaction time, 7 h; substrate molar ratio, 7.5:1 ethanol:soybean oil; enzyme content, 15% (g enzyme g(-1) oil); added water, 4% (g water g(-1) oil). The experimental yield conversion obtained under these conditions was 96%, which is very close to the maximum predicted value of 94.4%. The reaction time-course at the optimal values indicated that S h was necessary to obtain high yield conversions. CONCLUSION: A high yield conversion was obtained under the optimized conditions, with relative low enzyme content and short time. Comparison of predicted and experimental values showed good correspondence, implying that the empirical model derived from RSM can be used to adequately describe the relationship between the reaction parameters and the response (yield conversion) in lipase-catalyzed biodiesel synthesis. (c) 2008 Society of Chemical Industry.