In this work, in situ FTIR thin-layer spectroelectrochemistry has been employed at a flat mercury-based electrode to obtain, uniquely, the spectrum of the reduced form of flavin adenine dinucleotide (FAD) in neutral (pH 7) solutions. The flat mercury surface was achieved by the open-circuit amalgamation of gold and it was found to display the advantageous electrochemical properties, including the high hydrogen overpotential, of mercury. This electrode facilitated the determination of the difference in infrared absorbance resulting from the reduction of a solution of FAD. The spectrum of reduced FAD was recovered from the difference spectrum by adding to it the conventionally determined solution spectrum of the stable oxidized form of the redox couple. The observed shifts in band frequencies for the reduced vs. oxidised forms of FAD are consistent with the prior band assignments by normal coordinate analysis and with the previously proposed sites of the redox chemistry of FAD.