In Brazil, the National Agency of Petroleum, Natural Gas and Biofuels (ANP) is a regulatory agency responsible for monitoring the distribution and sale of fuels. After the introduction of biodiesel in Brazil, problems began to emerge as well as questions regarding the quality of diesel fuel in the country. One of the problems is related to the determination of biodiesel content in the diesel available in fuel stations. This work shows that biodiesel prepared from different feedstock during storage suffers oxidative degradation leading to different oxygenated compounds. The presence of these biodiesel oxidation products affects the accuracy of the result provided by the standard method EN 14078 in relation to the content of biodiesel in diesel. The result showed determination errors of almost 100%. Indeed, a B5 blend after accelerated oxidation was determined using EN 14078 as B10. Another problem observed using EN 14078 was that in countries like Brazil, where different oleaginous or alcohol are used to produce biodiesel, it is mandatory to calibrate the method with the same biodiesel present in diesel blends. Indeed, it was observed important changes in the position of the stretching related to the ester carboxyl group according to the raw materials used to prepare the biofuel, leading to significant errors in the biodiesel content determination. (C) 2014 Elsevier Ltd. All rights reserved.