In this paper we describe the free-radical-promoted grafting of maleic anhydride onto poly[bis(4-ethylphenoxy)phosphazene] carried out in solution under variable experimental conditions. It was found that this reaction depends on several different experimental parameters, i.e. the percentage of maleic anhydride in the system, the type and the thermal stability of the exploited peroxide initiators, the solvents used to run the grafting experiments, the amount of molecular oxygen present in the reaction medium, the heating time, and the reaction temperature. The grafting of maleic anhydride onto poly[bis(4-ethylphenoxy)phosphazene] induces profound modifications both in the physical (viscosity and glass transition temperature) and in the chemical (reactivity) properties of the phosphazene polymer and opens up unexpected perspectives to further; functionalization reactions and compatibilization processes for the phosphazene polymer.