A series of lipase-catalysed transesterification experiments were carried out to study the effect of the presence of free fatty acids on synthesis reaction rate and the stability of the biocatalyst, and also to elucidate the underlying process, which remains a subject of debate. Based on the results, the reaction rate and biocatalyst stability increased with increasing content in free fatty acids of the reaction mixture. Also, tests carried out with a mixture of triolein and linoleic acid revealed that the transesterification is a combination of direct alcoholysis of triacylglycerols and a two-step reaction involving hydrolysis of acylglycerols and further esterification of previously released free fatty acids. The time course of triacylglycerols and diacylglycerols revealed that the enzyme is similarly selective for both types of substrate. (C) 2015 Elsevier Ltd. All rights reserved.