A rig has been built to simulate the erosion-corrosion conditions in BFBC (Bubbling Fluidised Bed Combustion) power plants. In this particular test, 304 stainless steel tubular specimens were exposed for 3 weeks in air at 550 degreesC. The degradation was quantified, as a function of the impact angle, from measurements of the roundness of the specimens. The maximum eroded depth varied within 25-80 pm, depending on position in the bed. The compositions, thicknesses, morphologies and structures of the oxides, formed on both eroded and protected areas, were determined by AES, SEM/EDS and X-ray diffraction. The thicknesses of the residual oxides on the eroded surfaces and on the protected areas were 1-3 mum and 0.1-0.3 mum, respectively. The compositions of the eroded oxides were significantly different from the non-eroded oxides. The mechanism of erosion-oxidation differs considerably from the mechanism of oxidation only. It is concluded that defects in the oxide scale, formed by impacts of the particles, act as short-circuit diffusion paths and a mechanism based on this conclusion is proposed.