The existence of three polymorphs of poly(diethylphosphazene) has recently been established by MAS NMR and X-ray diffraction characterization. In the present work, the mechanism of motion of the ethyl side groups in the high-temperature modification of poly(diethylphosphazene) is identified and compared to the arrangement of side groups in the low-temperature modification. For this purpose, a rotor-synchronized triple resonance NMR experiment, called TRAPDOR, was applied for the qualitative evaluation of the average distances between carbons and nitrogens in the polymorphs. The agreement between these qualitative average distances and the molecular modeling supports the hypothesis that the ethyl groups undergo a complete rotation about the P-CH2 bond in the high-temperature modification.