Biodiesel, an alternative biodegradable diesel fuel, is derived from triglycerides by transesterification with alcohols. In this work, the transesterification of sunflower oil was investigated in supercritical methanol and supercritical ethanol at various temperatures (200-400°C) at 200 bar. The rate coefficients and the activation energies of the reaction were also determined. Biodiesel was also enzymatically synthesized in supercritical carbon dioxide. The effect of enzyme loading, oil to alcohol ratio, reaction time and temperature was investigated. While nearly complete conversions were obtained for the thermal reactions in supercritical methanol and ethanol, only 30% conversions were obtained in the enzyme-catalyzed reactions in supercritical carbon dioxide. © 2004 Elsevier Ltd. All rights reserved.