The influence of crystalline morphology on the dynamic mechanical relaxation properties of poly(ether ether ketone) (PEEK) has been investigated for the glass-rubber (alpha) and sub-glass (beta) relaxations; a series of both cold-crystallized and melt-crystallized specimens were examined. The presence of crystallinity had a marked influence on the glass-rubber relaxation characteristics of PEEK owing to the relative constraint imposed on the amorphous-phase motions by the crystallites. Above T(g), a progressive relaxation of rigid amorphous-phase material was evident, as well as a small incremental decrease in modulus reflecting the onset of the low-temperature melting component. The sub-glass mechanical relaxation was bimodal, comprising a lower-temperature (beta1) component, which originated in the bulk of the amorphous material, and a higher-temperature (beta2) component, which originated in organized regions of the amorphous phase (i.e. at the crystal-amorphous interphase). Both the dynamic mechanical and corresponding dielectric results displayed morphological sensitivity in the beta relaxation region, with the mechanical results encompassing motions of a more complex (i.e. cooperative) nature.