Disclosed is a method of monitoring and optimizing the concentration of a denaturant composition in a fuel ethanol. The method includes adding a known amount of the denaturant composition to the fuel ethanol to create a treated fuel ethanol. A measured spectroscopic absorbance or transmittance signal provides information for determining the concentration of the denaturant composition in the fuel ethanol. A component in the denaturant composition is capable of providing the spectroscopic absorbance or transmittance signal or capable of being chemically derivatized to provide a spectroscopic absorbance or transmittance signal. Based upon the measured spectroscopic absorbance or transmittance signal, the concentration of the additive composition in the fuel ethanol may be adjusted.