Mössbauer spectroscopy and X-ray diffraction were used to establish the composition of the rust formed on pure iron and weathering steel after exposure to several wet-dry cycles in an SO2-polluted atmosphere. α-FeOOH poorly crystallized and quasi amorphous ferrihydrite are identified as the main corrosion products. The rust has different particle size for iron and weathering steel samples. © 1991 J.C. Baltzer A.G. Scientific Publishing Company.