High-resolution carbon-13 and deuterium n.m.r, experiments have been used to examine the effects that low-molecular weight additives have on the secondary relaxation processes in polyethylene terephthalate (PET). These experiments have revealed that both the phenyl rings and the carbonyl groups in PET undergo small angle oscillations at temperatures below the glass transition temperature. It would, however, appear from the deuterium n.m.r. experiments that the phenyl rings also undergo rapid 180 flips, which appear to be closely related to the relaxation peak that is observed in dynamic mechanical experiments. Extremely little motion has, however, been observed in the ethylene glycol units and it has therefore been concluded that this group does not contribute to the secondary relaxation processes. Finally it has been observed that by blending low-molecular weight additives into the polymer it is possible to significantly reduce the number of the phenyl ring flips in the polymer and thereby suppress the relaxation peak.