Local aggregate structures in ethylene-methacrylic acid (E-MAA) and ethylene-acrylic acid (E-AA) ionomers were studied using extended X-ray absorption fine structure spectroscopy (EXAFS). In the case of pure zinc neutralization, E-AA and E-MAA ionomers showed very minor differences in EXAFS spectra. These small differences were statistically significant, however, and almost certainly a result of the structural differences between the acid repeat units. Overneutralization with zinc acetate dihydrate caused significant changes in the EXAFS pattern which could not be attributed to a linear combination of EXAFS spectra from the ionomer and zinc acetate dihydrate. Coneutralization with either lithium or sodium also changed the local environment around zinc. Zinc/sodium ratios were systematically varied in both E-AA and E-MAA. In the former, the local environment around zinc changed as the Zn/Na mole ratio changed from 1:0 to 1:1; at ratios between 1:I and 1:9 the local environments were identical. Therefore, at low Zn/Na ratios only one type of zinc-containing aggregate exists in the system, and this aggregate probably contains both sodium and zinc. A partial list of properties for this mixed-cation aggregate was generated, and possible structures were proposed. For E-MAA ionomers, zinc local environment changes continuously as Zn/Na ratio changes. Hence, more than one type of aggregate exists at all Zn/Na ratios, and the relative distribution of these aggregates changes with this ratio. The implications of the differences in aggregate structure on the rheological behavior of these mixed-cation systems are discussed.