The lattice parameters and spontaneous magnetizations of the pseudobinary compounds (R = Tb, Dy) are reported. The analysis of these results revealed the mutability of the Ce 4f ferromagnetism. The change of the degree of delocalization of the Ce 4f electron is used to analyse the changes of the lattice parameter and the spontaneous magnetic moment. A metamagnetic phase transition can be observed for each of the systems investigated, , when the Ce concentration x is about 0.5. The critical phase field increases with the Ce concentration, and the itinerant-electron model has been applied to analyse the tendency towards change of the critical phase field with x.