The oxidation stability of biodiesel fuels is a property specified in the European and American biodiesel quality standards. It is measured by the Rancimat test, whose parameters and final result (an induction period) are subjected to a continuous discussion among different committees, associations, distributors and final users. This study evaluates the stability of three biodiesel fuels derived from different feed-stocks, assessing the effect of the test temperature and the concentration of a commercial oxidation stability enhancing additive. The results showed a drastic decrease (in the range of one order of magnitude) in the induction period when the test temperature increases from 110 degrees C to 130 degrees C. This finding makes impossible to achieve the threshold currently imposed in the standards at 130 degrees C, even after a massive additive addition. The effect of the unsaturation degree and the previous use of the feedstock in cooking processes are also addressed. An experimental model, adjusted with statistical tools, is proposed to estimate the induction period as a function of the test temperature, the additive concentration and the origin of the biodiesel, the last considering the unsaturation degree and the previous use of the raw material. (C) 2011 Elsevier Ltd. All rights reserved.