In the present work the alkaline transesterification of sunflower seed oil with methanol and ethanol, for the production of biodiesel fuel was studied. Both conventional and in situ transesterification were investigated using low frequency ultrasonication (24 kHz) and mechanical stirring (600 rpm). Use of ultrasonication in conventional transesterification with methanol gave high yields of methyl esters (95%) after a short reaction time (20 min) similar to those using mechanical stirring. Use of ultrasonication in conventional transesterification with ethanol gave similar yields to those using mechanical stirring but significantly lower than respective yields using methanol. In the in situ transesterification the use of ultrasonication and mechanical stirring led to similar high yields (95%) of methyl esters after approximately 20 min of reaction time. In the presence of ethanol use of ultrasonication led to high ester yields (98%) in only 40 min of reaction time while use of mechanical stirring gave lower yields (88%) even after 4 h of reaction time. In situ transesterification gave similar ester yields to those obtained by conventional transesterification being an alternative, efficient and economical process. In all cases a concentration of 2.0% NaOH gave higher ester yields. Reaction rate constants were calculated, using first order reaction kinetics, to be equal to 3.1 x 10(-3) s(-1) for conventional transesterification using methanol and 2.0% NaOH, and 9.5 x 10(-4) s(-1) using ethanol. (C) 2007 Elsevier B.V. All rights reserved.