The equilibrium position in lipase mediated esterification of various fatty acids and butanol was studied. The influence of the chain length and the presence of unsaturations in the fatty acids on the equilibrium position was measured and predicted. To predict equilibrium position the program TREP extended (TREPEX) based on the UNIFAC group contribution method was used. Using an equilibrium constant of 35, calculated on the basis of thermodynamic activities, the equilibrium position between butanol and saturated and/or unsaturated fatty acids with different chain lengths can be predicted. The eater mole fraction at equilibrium increases with the fatty acid chain length, and for fatty acids with the same carbon number, the highest values are found for unsaturated fatty acids. For reaction systems containing two saturated fatty acids, a slightly higher mole fraction is obtained for the fatty acid with the higher chain length, while for mixtures consisting of saturated and unsaturated fatty acids, the mole fractions of the unsaturated esters are lower than those of the saturated ones, regardless the chain length of the fatty acid. These experimental results are in goad agreement with the calculations with TREPEX. (C) 2000 John Wiley & Sons, inc.