The fireside corrosion of a martensitic P92 steel with mixed sulfate deposit (Na2SO4/K2SO4 = 8:5, by weight) in the air at 650 °C was studied via X-ray diffraction, scanning electron microscope and electron probe microanalyzer and X-ray photoelectron spectroscopy. The results showed that P92 was highly susceptiblr to fireside corrosion attack. There were three corrosion layers: the Fe2O3 oxides in the outermost layer, the spinel oxides in the middle layer, and the mixed bands with oxide-sulfide between the inner oxide and the substrate. Sulfide particles were also distributed at the interface of martensitic lath in the matrix. There were different corrosion mechanisms in different corrosion layers. On the surface, basic fluxing of the oxide film destroyed the protective performance of the oxide film. At the forefront of the corrosion layer, the existence of sulfide and oxide mixed band and sulfide particles accelerated the mass transfer process and promoted the rapid oxidation of sulfide, making the oxidation rapidly penetrate the matrix. In nodular oxide, the formation of low melting point complex sulfate accelerated the mass transfer and oxidation process. [ABSTRACT FROM AUTHOR]