[en] Composition effect of high-pure molybdenum and vanadium steels on the processes of secondary hardening and role of impurities in the processes of secondary hardening are investigated. Electron-diffraction study of the structure at different ageing stages is carried out. It is stated that in those cases, when carbide-forming ability of impurities is higher, than carbideiforming ability of the main alloying element, presence of such impurities in the alloy results in an essential increase of hardness curves as compared with alloys without impurities; secondary hardness is the result of formation of precipitations coherent to the matrix and it is observed independent from intrinsic structure of the precipitations (mixed zones or special carbides)