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2 Y4CrlMo steel and 1 YCr XMO steel have been widely been used for hydro-processing units such as hydro-desulphurising and hydro-cracking reactors. These reactor pressure vessel steels have a potential for temper embrittlement that leads to toughness degradation and a reduction of the critical flaw size for brittle fracture. These steels are also susceptible to hydrogen embrittlement, especially in aged steels where cracks may propagate in the base metal up to the critical flaw size. A vessel with adequate toughness when originally constructed may therefore embrittle during service and such changes may require pressure restrictions during start-up and shut-down.A survey of the literature shows composition to be the controlling parameter for both temper embrittlement (TE) and hydrogen embrittlement (HE), in-particular the presence of residual impurity elements such as P and the presence of elements such as Mo which nullify the effect of impurity segregation.Much information is available to describe embrittlement phenomena for Cr-Mo steels. This paper reviews the mechanisms of TE and HE and describes a microstructural characterisation route which subsequently allows the structural integrity of potentially embrittled vessels to be examined for the purposes of remaining life assessment and plant life extension.