Initial SO2-induced atmospheric corrosion of iron has been followed in situ by three highly surface sensitive and complementary techniques, infrared reflection absorption spectroscopy, quartz crystal microbalance, and atomic force microscopy. The resulting corrosion attack was local in nature and resembled what are called sulfate nests, frequently observed on steel naturally exposed outdoors. The conclusions drawn challenge the established model for the formation and growth of sulfate nests: SO2 alone in humid air is not a sufficient prerequisite for sulfate nest formation. Only when an oxidant such as NO2 or O-3 is added to the corrosive atmosphere, can sulfate nests be detected. The conditions and formation of sulfate nests and other forms of local corrosion attacks are discussed in view of all in situ observations generated. (C) 2004 The Electrochemical Society.