In this paper the radical-induced solution grafting of diethyl maleate onto poly[bis(4-ethylphenoxy)phosphazene] is reported, together with the possible parameters that can influence this reaction, i.e., the maleate, molecular oxygen, and dicumylperoxide concentration in the reaction mixture, the solvent, time, and temperature of the process. A possible mechanism of this process is inferred on the basis of literature data concerning analogous grafting processes carried out onto polyolefins. The importance of the decrease of the intrinsic viscosity of the reaction mixture that takes place during the grafting reaction is also discussed in terms of chain scission and molecular weight degradation of the phosphazene macromolecule. The possible implication of the final phosphazene copolymers containing variable quantities of grafted diethyl succinate groups, poly[bis(4-ethylphenoxy)phosphazene]-g-diethyl succinate, in blending processes with commercial, carbon-backboned macromolecules (polyesters and polycarbonates) is stressed.