In situ infrared reflection spectroscopy has been used to probe the concentration change of the perchlorate anion in the electrical double layer at low-index single crystal gold electrodes. Cyclic voltammetry was used to estimate the amount of anionic charge in the diffuse layer, assuming the absence of specific adsorption near the potential of zero charge (pzc). A correlation between the integrated intensity of the perchlorate band at 1109 cm(-1) and the surface excess of the anion in the diffuse layer estimated by the Gouy-Chapman theory was found for sigma(m)<10 mu Ccm(-2). At higher positive charge densities on the metal, systematic deviations were observed. A strategy to determine the pzc at the higher concentration of perchloric acid (1 M), which is not available from electrochemical techniques, is presented. This involves the previous determination of the effective extinction coefficient of the perchlorate anion in the double layer. This parameter was determined from the analysis of integrated IR bands at a lower concentration (0.1 M), for which the cyclic voltammogram provides the value of the pzc, Based on these results it is shown that SNIFTIRS data can be used to confirm the location of the pzc in both dilute and concentrated solutions under circumstances where more traditional techniques cannot be used.