After the development and consolidation of automotive ethanol technology in the last thirty years, Brazil is starting the development of biodiesel technology pushed by the coming oil crisis. Since, the Brazilian biodiesel law sets the percentiles of biodiesel mixed in diesel oil 2% after 2008, and to 5% after 2013, the development of a method to measure such mixture percentiles becomes necessary. We present a method of measurement that covers the full range of mixtures (0-100%) of biodiesel in diesel oil using mid infrared spectroscopy. We show that the carbonyl (C=O) absorption peak, which is present only in biodiesel, presents power law proportionality with mixture percentile of 0.889 for peak area and 0.841 for peak height, respectively. Therefore it becomes possible to measure mixture percentiles of biodiesel in diesel oil, even at very low concentrations, allowing quality control and law enforcement. (c) 2006 Elsevier Ltd. All rights reserved.