Platinum deposited onto high and low surface area graphites are used as model catalysts in studying the irreversible catalyst deactivation observed previously in the aerobic oxidation of L-sorbose to 2-keto-L-gulonic acid. The corrosion of platinum in the neutral aqueous solution was followed in the liquid cell of a scanning tunnelling microscope. Leaching of platinum was well detectable in the time scale of 1-2 h. Depending on the nature of graphite support and the Pt content, a considerable fraction or even all Pt dissolved during sorbose oxidation, as evidenced by cyclic voltammetry. The strikingly high rate of Pt dissolution into the neutral aqueous medium under ambient conditions is attributed to the excellent chelating properties of the polyhydroxy carboxylic acid product.