An analytical method based on size-exclusion chromatography allowing to determine simultaneously the total amounts of triglycerides, diglycerides, monoglycerides, fatty acid methyl esters, free glycerol and methanol in samples of the transesterification reaction of sunflower oil with methanol is presented. Only one chromatographic peak was obtained for each kind of compounds, which resulted in an easy and accurate quantitation of these substances. Analyses were carried out at room temperature with samples directly withdrawn from the reactor and subjected only to minimal pretreatments in order to short-stop the reaction. The analytical method was used to monitor the synthesis of biodiesel from sunflower oil and methanol in a series of reactions carried out at 323 K in a mechanically stirred batch tank reactor. The effects of the concentration of homogeneous catalyst (NaOH and KOH) and the methanol/oil molar ratio used on the selectivities to the various products were studied. The influence of two distinct sampling procedures on the experimental results has been also investigated. (c) 2006 Elsevier B.V. All rights reserved.