In this work, an experimental system was designed to allow the online monitoring of a chemical reaction in continuous flow leading to biodiesel synthesis using a portable digital refractometer. The proposed method was applied to the monitoring of the methanolysis of soybean oil using KOCH3 as the catalyst at temperatures from 30 to 60 degrees C, allowing data acquisition in a relatively simple, reliable and cheap fashion. It was also possible to identify, discriminate and monitor the mixing (emulsification) and the reaction steps, with the former being the rate determinant. The methanolysis reaction is better represented by a zero-order kinetic scheme than by a pseudo-first-order scheme; the activation energy was determined to be (31.3 +/- 1.8) kJ mol(-1). This behavior suggests that methanolysis with alkaline catalysts, usually considered as a homogeneous process, should in fact be assumed to be heterogeneous. Therefore, the rate of mixing controls the reaction kinetics and is a key factor in decreasing the transesterification time. (C) 2014 Elsevier Ltd. All rights reserved.