Poly(diethylphosphazene) (PDEP), contrary to other polyphosphazenes, is hardly soluble in standard organic solvents and presents a melting point 80 degrees C higher than that of the dimethyl analog. Stimulated by this specificity, we have investigated the polymorphic behavior of PDEP and its cyclic trimer by X-ray diffraction, MAS NMR, and differential scanning calorimetry. The crystal structure of the native room temperature form of the cyclic trimer has been solved. The resulting arrangement of the ethyl groups has provided the basis to interpret the MAS NMR spectra of the trimer itself and of the polymer in the native modification, which present remarkable similarities. The MAS NMR data show analogous averaging of the ethyl side-chain conformations in the modifications obtained heating samples above the transition temperature of both the trimer and the polymer. X-ray and thermal data indicate that extensive tridimensional order must be maintained in these forms. Both modifications of PDEP obtained after thermal treatment present very significant side-chain mobility, coupled, in one of them, with only minor perturbations of the main-chain conformation and of the packing as compared to the native form. These unusual features imply for the disordered phases of PDEP a model substantially differing from the hitherto reported polyphosphazenic mesophases, which may account for the specificity of crystalline PDEP.