NOVELTY - Monitoring and optionally optimizing the concentration of denaturant composition in fuel ethanol involves adding known amount of denaturant composition to fuel ethanol to form treated fuel ethanol, measuring spectroscopic absorbance or transmittance signal for component in the treated fuel ethanol at a point, determining the concentration of composition in the treated fuel ethanol based on the measured spectroscopic absorbance or transmittance signal, and optionally adding known additional volume of fuel ethanol or additional known amount of composition.    USE - The method is used for monitoring and optionally optimizing concentration of denaturant composition in fuel ethanol. The method is used in production process where denaturants and/or denaturant and corrosion inhibitor(s) are added and may also be implemented at any stage of packaging and shipping processes. The method is applicable to different sampling techniques including grab samples, sidestream and inline measurements, and measurements taken from bulk container or vessel.    ADVANTAGE - The method allows fuel ethanol producer to easily minimize costs of production by adjusting formulations based on raw material costs and to maximize the quality and value of fuel ethanol product. The method is suitable for all manner of fuel ethanol production, compatible with all grades of fuel ethanol mixtures, and suitable for use with different fuel ethanol denaturants.    DETAILED DESCRIPTION - Monitoring and optionally optimizing the concentration of denaturant composition in fuel ethanol involves:    (A) adding known amount of denaturant composition to fuel ethanol to form treated fuel ethanol, in which the known amount is calculated to provide optimum concentration range for composition in the treated fuel ethanol, and the composition comprises component(s) that is either inherently provides spectroscopic absorbance or transmittance signal or can be chemically derivatized to provide spectroscopic absorbance or transmittance signal;    (B) measuring spectroscopic absorbance or transmittance signal for component in the treated fuel ethanol at a point;    (C) determining the concentration of composition in the treated fuel ethanol based on the measured spectroscopic absorbance or transmittance signal;    (D) optionally diluting the treated fuel ethanol by adding known additional volume of fuel ethanol if the determined concentration is above the optimum concentration range, in which the additional volume is calculated to bring the concentration of composition in the treated fuel ethanol into the optimum concentration range;    (E) optionally adding additional known amount of composition if the determined concentration is below the optimum concentration range, in which the additional known amount is calculated to bring the concentration of composition in the treated fuel ethanol into the optimum concentration range; and    (F) optionally repeating steps (A)-(E) until the determined concentration of composition is within the optimum concentration range.    An INDEPENDENT CLAIM is included for a digital storage medium comprising computer-executable instructions operable to execute the method.