Results of investigations of the mechanism of iron oxidation in atmosphere containing sulfur dioxide are presented. Experiments were carried out both by the platinum marker method and by means of the two -stage oxidation method using sulfur dioxide labeled with the35S radioisotope. SO2 partial pressures applied were 0.03 and 1 atm at 800°C. The reaction occurred by outward diffusion of iron ions at both sulfur pressures. In addition to cation diffusion there was also some inward sulfur penetration which occurred, however, not by volume diffusion but by short-circuit paths. At the lower SO2 partial pressure these paths are probably discontinuities in the scale (microfissures), whereas, forp = 1 atm, the inward penetration paths are probably large cavities produced by dissociation at the edges of the specimen when the reaction time is long enough.