The paper presents results of microstructure and mechanical testing examinations performed using optical and transmission electron microscopy, tensile tests and Charpy tests on 10CrMo9-10 and 13CrMo4-5 steels, before and after they were long-term operated at elevated temperatures in a steam heater. In the 10CrMo9-10 steel, the optical microscopy detected a degradation of original bainite that was accompanied by the formation of ferrite, precipitates and micropores. The transmission electron microscopy revealed that the precipitates are M 23 C 6 and M 7 C 3 type carbides, which are located mainly at the boundaries of former austenite grains, and M 3 C type carbides, which appear inside the grains. The 13CrMo4-5 steel contained a relatively high amount of ferrite in the ferritic-bainitic/perlitic microstructure already in the originally state. The degradation of the microstructure was less serious than for the 10CrMo9-10 steel. The thermal treatment of the 13CrMo4-5 steel led mainly to the precipitation of carbides. The M 23 C 6 and M 7 C 3 type carbides form in perlitic-bainitic areas, while M 3 C and M 6 C type carbides precipitate in ferrite. The higher density of the grain boundary precipitates in the long-term operated 10CrMo9-10 steel facilitated the formation of creep-induced micropores and contributed to the hardness reduction.