Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) was applied to determine the acid dissociation constant for bisulfate adsorbed at the Pt(111) electrode surface. The experiments were carried out in NaF + HF buffer solutions with pH varying between 0.4 and 6. 1. The SNIFTIR spectra were used to identify adsorbed bisulfate and sulfate ions and to monitor changes in the concentration of the adsorbed species with pH. The pK(a) for the adsorbed bisulfate was determined from the changes of the intensity of IR bands assigned to sulfate and bisulfate with pH. The values of pK(a) depend on the electrode potential and their values range between 3.3 and 4.7. They are significantly higher than the value of pK(a) in the bulk of the solution. The magnitude of pK(a) indicates that bisulfate is the predominantly adsorbed species at pH < 3.3 and sulfate is the predominantly adsorbed species at pH > 4.7.