Adsorption of the sulfuric acid anion (HSO4- or SO42-) has been studied on Pd(S)-[n(111) x (111)] electrodes (n = 2, 3, 5, 9, 20, infinity) using in situ infrared reflection absorption spectroscopy (IRAS). A single band is observed around 1200 cm(-1) on all the electrodes. The band is assigned to the SO stretching vibration of the sulfuric acid anion adsorbed with three- or onefold geometry. This result differs from the case of Pt-stepped surfaces on which two IRAS bands are observed around 1200 and 1100 cm(-1). The maximum coverage of the sulfuric acid anion is enhanced with the increase of the terrace width. The surfaces with n more than 3 have similar IRAS band shifts (dv/dE). I'd-stepped surfaces, for which the terrace is wide enough for the anion adsorption, adsorb the anion on the terrace rather than the step.