Biodiesel synthesis (transesterification of triglycerides of higher fatty acid to methyl and ethyl esters) from castor oil and linseed oil using methanol and ethanol was investigated at subcritical and supercritical conditions of methanol and ethanol from 200 to 350 degrees C at 200 bar. The effect of molar ratio of alcohol to oil, temperature, and time was investigated in supercritical methanol and ethanol. The kinetics of the reaction was first order, and the activation energies were determined from the temperature dependence of the rate coefficients. Biodiesel was also synthesized enzymatically with Novozym 435 in supercritical carbon dioxide (ScCO2). The effect of various parameters such as enzyme loading, alcohol to oil molar ratio, temperature, and time was investigated in these systems. A simplified model, based on the Ping Pong Bi Bi with competitive inhibition mechanism, was proposed to describe the enzymatic transesterification kinetics for castor oil with methanol and ethanol.