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a b s t r a c tThere are rather few quantitative data on the microstructure of the 9-12%Cr heat resistant steels after long-term creep. This paper presents results of the quantitative measurement of the size of MX precipitates, subgrain size and dislocation density in a P91 steel that had been creep tested for 113,431 h at 600 • C. The same measurements were conducted in the same P91 steel in the as received conditions.Transmission electron microscopy investigations were conducted using thin foils and revealed a decrease in dislocation density and an increase in subgrain size after creep exposure. MX carbonitrides are very stable during thermal and creep exposure of P91 steel at 600 • C up to 113,431 h.Electron backscatter diffraction (EBSD) investigations also revealed a significant change in the substructure of the steel after creep exposure.