The ionic aggregates in an as-extruded and a recrystallized poly(ethylene-ran-methacrylic acid) ionomer melt neutralized with sodium have been imaged using scanning transmission electron microscopy (STEM). The recrystallized sample exhibits a macrophase-separated structure with three phases with distinct boundaries. Phase I does not contain aggregates > 1 nm, phase II contains small spherical aggregates of ∼2−15 nm in diameter, and phase III contains large aggregates with diameters ranging from ∼20 to 160 nm. Contrast reversal between bright field and annular dark field STEM images indicates that the aggregates are Na-rich. In contrast, the as-extruded sample is featureless on the STEM length scale. This study shows that thermal history has a major impact on ionomer morphology. X-ray energy dispersive spectroscopy (XEDS) using a 1 nm probe confirms higher Na contents in the ionic aggregates and detects Na in the matrix. XEDS also finds a significant amount of C in the ion-rich aggregates due to the incorp...