The morphology of ionic aggregates in semicrystalline Zn- and Na-neutralized polyethylene-ran-methacrylic acid) (EMAA) ionomer blown films has been explored with scanning transmission electron microscopy (STEM) and small angle X-ray scattering. The ionic aggregates of Zn-EMAA are spherical, monodisperse, and uniformly distributed in as-extruded pellets and blown films prepared at low and high blow-up ratio. Thus, although the biaxial stresses of film blowing are sufficient to alter the PE superstructure, the ionic aggregates in Zn-EMAA are unaffected. In contrast, the morphology of Na-EMAA as detected by STEM changes from featureless in the as-extruded pellets to a heterogeneous distribution of Na-rich aggregates in the blown films. This transformation in Na-EMAA morphology is consistent with our earlier study of quiescent annealing, suggesting that the morphological change is the result of thermal processing rather than the biaxial stresses of film blowing. (c) 2005 Wiley Periodicals, Inc.