Severe corrosion problems were encountered in a bubbling-fluidised-bed (BFB) boiler burning chlorine-containing fuels. The highest corrosion rates were observed in the tertiary superheater region-that is, in the hottest tube section. The steam outlet temperature of the boiler was 530 degrees C, corresponding to highest metal temperatures of 560-580 degrees C. Maximum measured corrosion rate exceeded 10 mm/year. The original material of the superheater tubes was chromium steel CrMoV 12 1 (grade X20) according to DIN 17175. Corroded tertiary superheater tubes were studied with a scanning electron microscope (SEM) and energy-dispersive X-ray (EDXA) and microspot analysis. All the corroded tubes showed a layered structure in the corrosion scale. High chlorine concentrations were always found in the corrosion front-the interface between the metal surface and the scale. The corrosion front was followed by an iron-deficient/chromium-rich layer, which in some cases could be used for measuring the thickness of the corroded area. A third layer, consisting of almost pure iron oxide, was found to form on the original tube surface, but below the layer of ash deposit. Melting of the deposits appeared not to be responsible for the high corrosion rates observed, since the original tube outlines could be traced in the corroded samples. Chlorine, on the other hand, seems to have played a crucial role in the corrosion process. The presence of chlorides at the metal/scale interface seems to have enhanced oxidation of the alloy dramatically Chlorine penetrates into the metal through grain boundaries, and accelerates the corrosion by destroying individual grains. Iron diffuses towards the metal surface and is subsequently oxidised to iron oxides. The main source of chlorine in the corrosion reactions is apparently the sulphation of alkali chlorides occurring in the deposits of fly-ash on the tubes. However, sulphur and alkalis were not found to participate directly in the corrosion reactions. The morphology of corrosion scale is similar to that observed in high-temperature oxidation. In this case, however, chlorine seems to act as a catalyst, enhancing the diffusion of iron at low temperatures. The history of the growth of corrosion scale was found to be preserved more precisely under the tube ties than elsewhere; therefore samples from those regions were used to study the causes of corrosion.