In situ infrared reflection spectroscopy has been used to probe the concentration change of the perchlorate anion in the electrical double layer at (100) Au. The results obtained show clearly that there is a correlation between the integrated intensity of the perchlorate band determined in a subtractively normalized interfacial Fourier transform infrared spectroscopic (SNIFTIRS) experiment and the surface excess of the anion in the diffuse layer predicted by the Gouy-Chapman theory. An expression that correlates the above two quantities is derived.