Coordination structure and oxidation−degradation were investigated for two types of ionomers, ethylene−methacrylic acid (5.4 mol %) copolymer partially neutralized with Co(II) and its complex salt with 1,3-bis(aminomethyl)cyclohexane [1,3-(H2NCH2)2C6H10] (BAC), by using infrared (IR) spectroscopy and gas chromatography−mass spectrometry (GC−MS). The IR spectrum for the Co(II) salt was reversible with temperature when the sample was sealed from air with KBr windows, but irreversible in air, showing the presence of oxidation−degradation. The decomposition products were identified to be mainly alcohol, ketone, and alkanecarboxylic acid derivatives by the use of GC−MS spectrometry. The Co(II) complex salt with BAC, on the other hand, showed a reversible temperature dependence of the IR spectra, even when the sample was heated in air above the melting point of poly(ethylene) crystallites (∼90 °C). The difference in thermal stability between the two types of ionomers was well explained by the coordination structures of Co(II) ions. Finally, the present study demonstrated that the asymmetric COO- stretching band for both ionomers significantly changes around a transition temperature (∼50 °C) of ionic aggregate regions, which gives evidence for an order−disorder transition model of ionic aggregate regions proposed by us.