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)(2)C6H10] (BAG), 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 CC-MS spectrometry. The Co(II) complex salt with BAG, 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 (similar to 90 degrees 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 (similar to 50 degrees C) of ionic aggregate regions, which-gives evidence for an order-disorder transition model of ionic aggregate regions proposed by us.