In the context of the heat treatment optimisation of steels used for nuclear reactor pressure vessel, the carbide precipitations in three model alloys: Fe-0.2 %C-1 %Mn, Fe-0.2 %C-1.5 %Mo and Fe-0.2 %C-1 %Mn-1.5 %Mo have been experimentally investigated and modelled.The precipitation sequences for times up to 2 months at 650 and 700°C have been determinedFe-0.2 C-1.5Mn: cementite onlyFe-0.2 C-1 Mo: cementite →cementite + M2C →M2C + ξ carbideFe-0.2 C-1 Mo-1.5 Mn: cementite →cementite + M2C + ξ carbideThe volume fractions in carbide were determined by Rietveld refinement of synchrotron X-ray diffractograms. This characterisation has confirmed the tendency for cementite dissolution in favour of other Mo rich carbides in Mo containing alloys.Similarly, PRISMA modelling of the carbide volume fractions predicts a tendency for cementite dissolution in Mo containing alloys. It has also enabled the relative contribution of some of the factors affecting ξ carbide precipitation to be understood.