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Based on available published data and experimental findings, the in-service embrittlement of materials has been analyzed. The contributions of thermal ageing and neutron irradiation to embrittlement of the base and weld metals of WWER-1000 reactor pressure vessels are discussed. Equations have been derived which describe the shift of the critical brittle temperature depending on the irradiation time and neutron fluence. For a nickel-rich weld metal a relation between the radiation embrittlement coefficient and the amount of alloying elements Ni, Mn, and Si has been defined.Keywords: embrittlement, reactor pressure vessel, shift of critical brittle temperature, irradiation, neutron fluence. Introduction. Investigations [1-25] of embrittlement behavior of WWER-1000 reactor pressure vessel (RPV) materials [the base metal (BM) is 15Kh2NMFA-A steel and the weld metal (WM) -the weld joints of steel 15Kh2NMFA-A] give rather conflicting findings. This applies primarily to the description of the shift of the critical brittle temperature ΔT k k as a function of the neutron fluence [1]: