In this work, virgin and sulfonated poly(ether ether ketone) films (PEEK and SPEEK, respectively) have been studied by dynamic mechanical analysis, modulated differential scanning calorimetry, wide-angle X-ray diffraction, birefringence, and optical microscopy. The properties of the unmodified polymer have been addressed to assess the original morphological characteristics and the changes induced by sulfonation. In general, the introduction of ionic groups in the polymer backbone alters dramatically the intrinsic properties of the parent material. The particular thermomechanical response exhibited by PEEK and SPEEK samples, characterized by a hysteresis loop, can be explained by the reversible and irreversible relaxation-orientation of the microstructure, even in the sub-T-g region. The results showed that the preparation conditions largely determine the nonequilibrium morphological features of both compression-molded PEEK films and solvent-cast SPEEK membranes. (c) 2005 Wiley Periodicals, Inc.