The metal-solution interface of a single-crystal Pt(100) electrode in the presence of specifically adsorbed sulfate ions is investigated by means of in situ FTIR spectroscopy. Sulfate adsorption starts at about 0.2 V vs. Pd/H-2 and goes through a maximum at ca. 0.5 V. Sulfate ions are adsorbed through two oxygen atoms presenting a C2v symmetry. The two strong bands observed at around 1100 and 1200 cm-1 are assigned to the symmetric S-O stretches of the resulting SO2 groups (coordinated and non-coordinated (respectively).