Recent studies have shown that for the older East and West water reactor pressure vessel steels there is a similar degradation in mechanical properties during equivalent neutron irradiation. In both cases with the identification of and a reduction in the deleterious element concentration the sensitivity of the steels to neutron irradiation was reduced. The deleterious elements which have been identified in the national databases as being responsible for the sensitivity are different. In the ‘Western’ case copper and nickel are identified and used in the current version of the USNRC Regulatory Guide while phosphorus and copper are identified in the Russian Codes. For the comparison neutron fluence is harmonized and the end-of-life fluence differences are considered. Annealing is a mitigating action for reducing the effect of irradiation on these steels and another coincidence in the similarity of the response of these steels is seen. The paper reviews the current understanding on the mechanisms of irradiation embrittlement of these steels and also the role of compositional variations in their irradiation sensitivity. Phosphorus concentration is seen as the key difference between East and West steels. When the phosphorus concentration is greater than about 0.012 wt%, then its effect is a dominating one with regard to irradiation effects.