Transesterification of vegetable oil with methanol, promoted by alkaline catalysts, has been tested in many different reactors and the reaction time to obtain a complete conversion, in some of these reactors, resulted in the range of few seconds. By using static mixers, micro-reactors, oscillatory-flow reactors, cavitation-inducing reactors, microwave reactors or centrifugal contactors, it is possible to obtain high biodiesel yields in a much shorter time than in stirred tank reactors. As these reactors are characterized by a very high liquid-liquid interface we can conclude that higher is the interface area shorter is the reaction time. This behavior cannot be described with the monophasic kinetic model reported in the literature. Therefore, we have developed a biphasic kinetic model that is able to describe any type of reactor such as: more or less stirred batch reactors, more or less efficient continuous structured reactors, micro-reactors etc. We have also experimentally observed that it is possible to obtain a complete conversion in less than 10 seconds by using a very efficient micro-mixer followed by a void settling tube of an opportune volume size. This reactor has been simulated also with the mentioned liquid-liquid biphasic model. This model is based on a reliable transesterification mechanism and takes into account both the intrinsic kinetic laws, mass transfer and reactants and products two phases partition. Some of the unexplained kinetic aspects, observed in batch runs, like the difference in the final conversions for different catalyst concentrations have been clarified with the biphasic model. This final conversion is due to catalyst deactivation and not to chemical equilibrium as suggested in the monophasic kinetic models normally employed in the literature. In this work, the developed biphasic approach will be applied on different kinds of laboratory reactors characterized by a different micromixing efficiency. The kinetic parameters for the intrinsic occurring reactions will be given and discussed. Moreover, the role of the mass transfer effect will be considered and the consequence of the methanol and glycerol partition on the conversion and yield will be described. By concluding, the possibility to produce biodiesel in a micro-device with residence times of few seconds will be experimentally shown and theoretically interpreted.