The thermal decomposition of fatty acid chains was investigated in different reaction conditions [temperature, reaction time and methanol-to-oil molar ratio ranges of 250-350 degrees C. 15-90 min and 13:1 to 53:1, respectively] during the transesterification of refined soybean oil using supercritical methanol in a batch reactor. No decomposition was observed at 250 and 275 degrees C, regardless of the reaction time and methanol-to-oil molar ratio used. However, for practically all the molar ratios tested decomposition became evident at 300 degrees C when the reaction time was longer than 60 min whilst at 325 and 350 degrees C similar behavior was observed almost from the first moments of the reaction (15 min). The molar ratio favored the formation of both saturated and unsaturated fatty acid methyl esters and had some protective effect on the thermal decomposition of the polyunsaturated esters. A new method for determining the degree of decomposition is proposed based on the area of a new chromatographic peak detected during the analysis of the reaction samples according to the standard UNE-EN 14105:2003. (c) 2012 Elsevier B.V. All rights reserved.