Abstract: Described herein are embodiments of a method and device for determining the relative amounts of biodiesel and diesel in a biodiesel/diesel blend without separating the biodiesel from the diesel. Embodiments of the method comprise providing a device for measuring absorbance of the blend, measuring absorbance of the blend at one or more wavelengths, and determining the relative amounts of biodiesel and diesel in the blend from the absorbance. Embodiments of the device comprise a light source and a detector for detecting light transmitted through a sample of a biodiesel/diesel blend. Typically the device includes a data analyzer for computing relative amounts of biodiesel and diesel in the blend. Some embodiments of the device further comprise one or more filters, which allow only light of a particular wavelength or wavelengths to pass through the filter.