The absorption of sulfate species was studied on well ordered Pt(111) surfaces at pH 2.8 and 0.23 using in situ external reflectance ir spectroscopy. Independently of the pH in the solution the spectra present two overlapped vibrational bands. A strong one is located at 1220-1280 cm-1 and is potential dependent. The second one is observed as a shoulder at 1190 cm-1. The analysis using group theory and the surface selection rule shows that only sulfate ions are adsorbed at both pH, suggesting a strong Lewis acid character of the Pt(111) surface. The randomly distributed sulfate species are relatively tightly adsorbed and present weak lateral interaction. At increasing coverage the sulfate adlayer becomes more organized, as attested by the observed band narrowing. The maximum adsorption is observed at 0.8 V vs. Pd/H-2 ie at potentials higher than that of the so called anomalous states in the voltammogram. A quadratic dependence of the applied electric field on the S-0 vibrational band of adsorbed sulfate is observed, suggesting a second order Stark effect.