Poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) (PEEK) selectively H-2 labelled in the ether-keto rings has been studied by H-2 NMR T-1 relaxation time measurements. Partially relaxed H-2 NMR lineshapes at 360 K indicate the predominant dynamic process as a phenyl ring pi flip. Computer simulations based on the model of a delta function for a rigid, crystalline phase and a log-normal distribution of phenyl ring flip rates for the amorphous phase, show that fits to a combination of a simple and stretched exponential are possible. However, accurate fits to the experimental H-2 Saturation recovery curve based on exponential and stretched exponential functions could not be obtained over the full temperature range of 260-421 K. Constraining the fit to the trends seen in the computer simulations, allowed fits with only slightly increased errors suggesting that experimental error combined with the flexibility of the stretched exponential leads to the poor fits. The acquisition of improved experimental data is severely hampered by the long crystalline phase T-1. An estimate of the fast flipping fraction was made on the basis of the intensity after 100 ms recovery in the H-2 saturation recovery curve. From the increase in the fast flipping fraction with temperature, an energy of free volume formation of 10 +/- 2 kJ mol(-1) can be estimated. Evidence was found for fraction of the rings in the amorphous phase not undergoing pi flips at the T-g. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.