Biodiesel samples containing rosemary, oregano and basil extracts, free of alcohol, were submitted to oxidative stability tests at temperatures of 110, 115, 120 and 125 degrees C. The rate constants and enthalpy (Delta H-double dagger), entropy (Delta S-double dagger) and the Gibbs free energy (Delta G(double dagger)) in the activated state were determined from the experimentally obtained induction periods for evaluation of efficiency of natural antioxidants. A simplex-centroid mixture design was applied and the predictive equations were optimised simultaneously resulting in values of 85.89 kJ mol(-1) for Delta H-double dagger, -32.25 J K-1 mol(-1) for Delta S-double dagger and 98.48 kJ mol(-1) K-1 for Delta G(double dagger) at ratios of 50% rosemary extract, 25% oregano and 25% basil. The results (Delta H-double dagger> 0; Delta S-double dagger< 0 and Delta G(double dagger)> 0) showed that, in the presence of natural antioxidants, the oxidation reaction was endothermic and not spontaneous. (C) 2015 Elsevier B.V. All rights reserved.