Abstract: This patent presents a method for measuring the concentration of biodiesel in a homogeneous biodiesel-diesel oil mixture using mid infrared radiation for use as fabrication monitor, quality control, law enforcement and multi-fueled vehicles. The method is characterized by the use of mid infrared absorption measurement in the range from 1870 to 1600 cm?1 (5347.6 to 6250.0 nm) which corresponds to the carbonyl group absorption peak (C?O) that is only present in biodiesel. Both intensity and area of the carbonyl absorption peak present power law dependence with the biodiesel percentile in the biodiesel-diesel oil mixture.