This paper presents an ultrasonic method capable of being applied in a process line for monitoring the soybean oil transesterification with methanol, catalysed by KOH. Four production routes had been studied, varying the concentration of catalyst (0.2% and 1.5% w/w) and the rotation period of the mechanical stirring (200 rpm and 520 rpm). A low-power, 1 MHz ultrasound signal was used with the pulse-echo method to interrogate the medium during the transesterification process, being the speed of sound the measurement quantity. Variation of this quantity was compared throughout the process with traditional and precise analytical methods (gas chromatography and nuclear magnetic resonance of hydrogen). Aliquots were collected every 5 min from the reactional medium, and the conversion rate was assessed thereafter. The measurement of ultrasonic velocity showed that the ultrasound could identify variations occurring in the reactional medium which are correlated to ester production. Complementary, results indicate that the ultrasonic velocity can aid in distinguishing among different biodiesels produced. Further, the reactional time of flight ratio is presented as an alternative method for monitoring the biodiesel synthesis in real time. As a final remark, it was possible to use the proposed ultrasonic method to monitor the transesterification reactions in real time, allowing this technique to be applied in the process line minimizing wastes of reagents and energy.