The microphase structure of noncrystalline poly(ethylene-co-13.3 mol % methacrylic acid) (E-0.133MAA) ionomers was investigated by using infrared (IR) spectroscopic, X-ray scattering, differential scanning calorimetric (DSC), and dielectric measurements. The noncrystallinity was confirmed by small-angle X-ray scattering (SAXS) and DSC studies, which has enabled a quantitative analysis of the SAXS ionic peak associated with ionic aggregates without being perturbed by the polyethylene lamellae peak. In 60% neutralized Na ionomer, it was revealed that almost 100% of MAA side groups including unneutralized COOH are incorporated into the ionic aggregates with an average ionic core radius (R-1) of similar to6 Angstrom . The dielectric relaxation studies showed that the ionic aggregates form a microphase-separated ionic cluster. Analysis of dielectric strengths indicated the most (similar to 90%) of the COONa groups are present in the ionic cluster. On the other hand, in the 60% neutralized Zn ionomer, both SAXS and dielectric studies indicated that the ionic aggregates with R-1 similar to 4 Angstrom are almost isolated and dispersed in the matrix; the formation of ionic cluster was not recognized. Similarly to partly crystalline E-MAA ionomers, all noncrystalline E-0.133MAA ionomers exhibited an endothermic peak at 320-330 K (labeled T-i) on the first heating, depending on the aging time at room temperature. Several factors that would be critical for the DSC T-i peak were discussed. It was concluded that the DSC T-i peak is certainly associated with changes of the state of ionic aggregate region.