We studied the adsorption and reaction of sulfur dioxide on clean and oxygen precovered platinum surfaces by in situ high resolution X-ray photoelectron spectroscopy and discuss earlier, contradicting results. On the clean surface we find flat lying and upright standing SO2, adsorbing in similar amounts at low temperature. Upon heating, the ratio changes strongly favoring the standing SO2; subsequently almost all SO2 desorbs. On the oxygen precovered surface the standing SO2 is preferably populated and even at low temperatures a direct reaction of SO2 to SO3 occurs. Upon heating, SO2 oxidation to SO3 and subsequently to SO4 is found. (C) 2010 Elsevier B. V. All rights reserved.