H-2 NMR experiments were performed on a selectively deuterated aryl-aliphatic copolyamide in order to investigate the local dynamics of the phenyl rings as a function of temperature. The activation energy of the phenyl ring flip motion, determined from the H-2 NMR experiments, was shown to be identical to the activation energy determined by dielectric relaxation for the beta transition. Such a result tends to indicate that the phenyl ring flips are part of the beta transition in the aryl-aliphatic copolyamide under study. (C) 2000 Elsevier Science Ltd. All rights reserved.