The development of a mathematical model that accurately describes the formation of fatty acid methyl esters (FAMEs) (otherwise known as biodiesel) from rapeseed oil and methanol in the presence of sodium hydroxide catalyst at temperatures in the range of 293-333 K is described. The model accounts for the existence of more than one phase at any given stages of the reaction and for the solubilities of the reacting species in various phases. The latter were obtained experimentally and are presented in the form of ternary phase diagrams. The model appears to give a good fit to the experimental data obtained using both batch and continuous flow reactors.