In situ FTIR spectroscopy has been used to study the potential dependent changes in anion structure and composition at the surface of Pt(111) electrodes in HSO4--containing solutions. From the infrared differential normalized relative reflectance data, the maximum rate of intensity changes for three infrared bands can be obtained. Two modes associated with the adsorbed anion and one mode assigned to solution-phase bisulfate all have extrema coinciding with the spike in the anomalous region of the cyclic voltammetry. The adsorbed species is not adequately described as either sulfate or bisulfate; the data are more consistent with an adsorbed H3O+-SO42- ion pair, possibly with the three unprotonated sulfate oxygens interacting with Pt sites. A weak band at 940 cm(-1) correlates with the 1200-1250 cm(-1) band and is assigned to the S-O(H3O+) stretch of the ion pair. A band at 1040 cm(-1), which disappears with increasingly more positive sample potentials, is assigned to the symmetric stretching mode for solution-phase bisulfate ions.