The composition and morphology of scales formed on iron in sulfur dioxide at 3×103 Pa and 1073 K have been investigated by means of various X-ray techniques of analysis (XRD, EDAX, EPMA) and scanning electron microscopy (SEM). The transport phenomena have been studied using platinum markers and tracers. The scales were composed of a sulfide and oxide mixture and grew by outward diffusion of iron. There was also some inward transport of sulfur and oxygen which occurred through, the discontinuities of the scale. These oxidants, as well as those originating from the dissociation of the scale, take part in the formation of the inner scale layer in the metal-consumption zone. The initial stage of the process proceeds mainly by the reaction of iron with SO2 molecules.