A study was conducted on the influence of high humidity and temperature on aluminum and copper sheet coated with ethylene–acrylic acid or ethylene–methacrylic acid copolymers. Infrared spectrometric and differential thermal analysis data indicate that a contributing cause to failure of the bond between the acrylic acid copolymers and copper is the solubilization of an underlying weak oxide layer under conditions of the test. No significant weakening of the bond of aluminum–adhesive copolymer was observed under high humidity. From oxygen uptake studies, thermogravimetric analysis in air, and infrared spectra of polymer bonded to metals, a general understanding of the thermal-oxidative stability of the ethylene–acrylic and ethylene–methacrylic copolymers has emerged. The acrylic acid copolymers are less stable in an oxidative environment than ethylene homopolymers and the ethylene–alkyl acrylate copolymers.