Biodiesel is normally obtained by transesterification of triglycerides with methanol in the presence of an alkaline catalyst. The reaction, performed in stirred tank reactors, requires 1-2 h of reaction time. As the reactants are immiscible, the reaction rate can be affected by mass transfer limitation. We have recently shown, that all methods favoring local micromixing can give place to high performances. At this purpose, we have recently developed a very simple laboratory device for testing the behavior of the mentioned reaction in microchannels of different sizes. This device is simply a tubular reactor filled with stainless steel spheres of different diameters. By opportunely changing the spheres' diameters it is possible to obtain microchannels in a range of 300-1000 mu m. However, in these reactors the void portion of the reactor is low and the productivity per volume is low, too. It is possible to obtain better results in terms of productivity by filling the tubular reactor with stainless steel wool, being in this case the void fraction about 0.9. In this paper, the performances obtained with this type of reactor are reported together with a discussion on the reaction mechanism in view of the future development of a kinetic biphasic model. (C) 2011 Elsevier B.V. All rights reserved.