Although available data do not report significant thermal ageing of reactor pressure vessel materials, this should be reconsidered in the perspective of long term operation where exposure time is significantly higher than the available reported experimental data. One of the element that is reported to promote thermal ageing degradation is phosphorus. The thermal ageing embrittlement susceptibility is assessed by submitting reactor pressure vessel steels to a step cooling heat treatment known to induce P-segregation to grain boundaries resulting in intergranular rather than transgranular cleavage fracture. Thermal ageing susceptibility was experimentally assessed on two typical reactor pressure vessel steels with P-content of 0.017% and 0.006%. The experimental data including tensile, Charpy impact and fracture toughness tests, show a very consistent picture of the various mechanical properties. Upon step cooling, intergranular fracture is promoted and P-content has a major effect on the thermal ageing susceptibility. However, quantitative assessment shows that the effects of step cooling on the mechanical properties of both steels in their as received condition are not sufficiently high to induce thermal ageing embrittlement and therefore their susceptibility to thermal ageing is low. Consequently, given the low P-content of most Belgian RPV materials, it can be concluded that they will be not prone to thermal ageing in the perspective of long term operation.