The effect of inert gases, nitrogen and carbon dioxide, on the oxidative degradation of low-density polyethylene appearing as colored spots has been studied during an extrusion process in competition with an antioxidant. Extrusion under inert gases significantly decreases the degradation level in the critical region of the process in comparison with classical extrusion under air. The effect of antioxidants on degradation during extrusion at a high temperature is weak. The main processes acting on this reduction of polymer oxidation and the origin of the color of degraded domains have been investigated. Energy-dispersive spectra of particles have confirmed that degradation is caused by thermooxidation. The nature of chromophore groups in degraded areas has been identified by IR microscopy. We found that beta-conjugated ketoenols are present inside colored spots and seem to be responsible for the color of degraded parts. Quantum calculations have confirmed that such chemical structures absorb visible light and create reddish and brown colors. (c) 2007 Wiley Periodicals, Inc.