The kinetics of the esterification of free fatty acids (FFA) in sunflower oil with methanol in the presence of sulphuric acid at concentrations of 5 and 10 wt% relative to free acids as catalyst and methanol/oleic acid mole ratios from 10: 1 to 80:1 was studied. The experimental results were found to fit a first-order kinetic law for the forward reaction and a second-order one for the reverse reaction. The influence of temperature on the kinetic constants was determined by fitting the results to the Arrhenius equation. The energy of activation for the forward reaction decreased with increasing catalyst concentration from 50 745 to 44 559 J/mol. Based on the experimental results, a methanol/oleic acid mole ratio of 60:1, a catalyst (sulphuric acid) concentration of 5 wt% and a temperature of 60 degrees C provided a final acid value for the oil lower than 1 mg KOH/g oil within 120 min. This is a widely endorsed limit for efficient separation of glycerin and biodiesel during production of the latter. (C) 2007 Elsevier Ltd. All rights reserved.