Selectively deuterated poly(ethylene-2,6d-naphthalene dicarboxylate) (PEN) was synthesized, obtaining a material in which either the ethylene groups (PEN-Ed1or the naphthalene ring (PEN-Nd6) was deuterated. By means of deuteron nuclear magnetic resonance (2H-NMR), the molecular mobility of the deuterated groups was investigated as a function of temperature. The spectra were measured by the solid echo method, applying different waiting times twThe longitudinal relaxation curves were obtained by determination of the maximum intensity of the echo as a function of twIt was found that in the amorphous regions of PEN, the molecular mobility of the ethylene groups is higher than in the amorphous regions of PET. In PEN crystals in the β-modification form, trans-gauche jumps of the ethylene groups with almost equal occupation of the trans and the gauche conformation take place already at 90°C. This is not the case in either PEN crystals in the a-modification form or in PET crystals. The rotational motion of the naphthalene ring, which according to dynamic mechanical analysis at 20 Hz starts at 60°C, could not be observed in the NMR spectra. This is explained by the high frequency applied in the NMR experiment.