The transesterification of soybean oil using calcium methoxide as solid base catalyst and the process parameters affecting the yield of biodiesel such as the catalyst concentration, methanol/oil molar ratio and the reaction temperature were investigated in detail. The results showed that the yield of biodiesel in excess of 90% was achieved within 90 min using 1% catalyst loading, 9:1 methanol/oil molar ratio and 65 degrees C reaction temperature. In addition, the effects of catalyst storage time on its activity for transesterification reaction and ultrasound at 20 kHz and 611 kHz frequencies on the yield of biodiesel were also evaluated. TGA-DSC and BET studies showed that the catalyst has good thermal stability and high surface area, respectively. Furthermore, the mechanism of this heterogeneously catalyzed transesterification system with and without ultrasonication was demonstrated to be a two-step reaction in which mass-transfer controlled regime is accompanied by kinetically controlled regime with both regimes following pseudo first order reaction kinetics. (c) 2012 Elsevier Ltd. All rights reserved.