Two techniques based on the measurements of the refractive index (n(D)) of the alcoholic phase at 20 degrees C (n(D,AL)(20)) and the speed of propagation of sound (u(s)) in the reaction mixture are presented for monitoring the methanolysis of vegetable oils. Application of these techniques for the real-time monitoring of the synthesis of biodiesel is described for the first time. Size exclusion chromatography has been used as reference method. Both n(D,AL)(20) and u(s) increase linearly with the oil conversion due to the contribution of the glycerol formed. Good correlations have been found whose coefficients depend on the initial methanol/oil ratio and in the case of u(s) also on the temperature Due to flow instabilities in the refractometer chamber, monitoring based on n(D) has been possible only off-line whereas the measurement of u(s) allowed an effective, fast, accurate and cheap real-time monitoring of the process. (C) 2013 Elsevier Ltd. All rights reserved.