Biofuels are of great topical interest, especially in the light of recent European directives. Biodiesel is both already available and thoroughly regulated. A critical aspect of the transesterification process, used in biodiesel production, is maximization of the contact surface between reagents. This is currently achieved using discontinuous processes with mechanical agitation, which however afford poor mixing efficiency. Ultrasound appears to be an effective method to enhance mixing intensity. The aim of this work was to assess the advantages offered by this technique, especially in terms of process velocity. For this purpose a small scale experimental apparatus for biodiesel production integrated with a modular ultrasound system was set up. Plant dimension and energy inputs were chosen to enable eventual industrial implementation. Plant performances were assessed as the velocity of oil conversion to fatty acids methyl esters. The product was analyzed according to EN 14105. In general, the efficiency of ultrasound in increasing reaction velocity was markedly affected by residual triglyceride content. Conversion rates in line with EN 14214 and ASTM D6751 standard specifications, which allow a maximum residual triglycerides content of 0.20%, were obtained with reductions in reaction time of about 20%. In less stringent conversion conditions, e.g. 3% triglycerides, the reaction time was roughly halved. Below 1% of triglycerides residues sonication should be interrupted and the process continued with conventional agitation. (C) 2011 Elsevier Ltd. All rights reserved.