Selectively deuterated ternary copolyesters of poly(ethylene terephthalate) (PET), poly(ethylenenaphthalene-2,6-dicarboxylate) (PEN) and poly(p-hydroxybenzoic acid) (PHBA) showing the molar composition 35:35:30 were synthesized. In these copolyesters the liquid crystalline state, as well as the isotropic state, can be frozen in by quenching the material from different temperatures. In this way it is possible to compare the molecular mobility in both states. By means of dynamic mechanical analysis it was shown in previous investigations that the glass transition temperature, T-g, of the liquid crystalline state is about 35 degrees C lower than that of the isotropic state. The molecular mobility of the ethylene groups and of the phenylene rings below and above T-g in both the liquid crystalline and the isotropic state is now investigated by means of deuteron n.m.r. It is shown that the trans-gauche jumps of the CH2 groups in the elongated chains in the liquid crystalline state can occur at lower temperatures than in the coiled chains in the isotropic state. As a consequence, the fraction of flipping phenylene rings in the liquid crystalline state is larger than in the isotropic state, when compared at the same temperature.