A molecular level understanding of the properties of polymer electrolytes can only be obtained through an identification of the types of ionic structures formed in these materials and their relationship to the motion of ions in these systems. Vibrational spectroscopy has proved to be a powerful tool in characterizing such local structures, and a detailed study of the infrared spectrum of a set of sulfonated PEO/aromatic polyesters is presented here. An analysis of the symmetric SO3- stretching mode revealed no detectable amounts of "free" SO3- ions in any of the ionomers, but bands due to ion pairs and aggregates were identified. The band due to aggregates increased in intensity relative to the band due to ion pairs as the temperature increased. Vibrational modes due to the poly(ethylene oxide) segments of the copolymers were characteristic of chains in the amorphous state. However, bands due to sequences of trans and gauche O-C-C-O-C-C-O conformations characteristic of the ordered state appeared to be favored upon complexation with ions, The interaction between cations and PEO portions of the chain "locks" the segments in their preferred structure to a large degree, so that compared to non-sulfonated copolymers there are relatively small changes in the relative intensities of conformationally sensitive bands with temperature.