Polyethylene (PE)-filled with carbon black (CB) is a prototypical composite that displays resistance switching. These materials can exhibit either a positive temperature coefficient (PTC) or negative temperature coefficient (NTC). The CB-filled semicrystalline polymer composites ideally need antioxidants, which stabilize the composites against thermooxidative degradation, because they should be resistant to the severe conditions of high temperature. The characterization of PTC materials is affected by the crystallinity of the polymer, and the crystallinity of the polymer is changed with thermal ageing treatment. Thermal ageing of PTC samples was conducted in an oven in the range 50-140degreesC, in air. The composites, containing 0.5-3% (by weight) Irganox 1076 (Ciba-Geigy), were irradiated under nitrogen at room temperature with different doses of gamma rays from a Co-60 source. The resulting composites were analyzed by differential scanning calorimetry, gel fractionation, X-ray diffraction, and dynamic mechanical analysis. The conductivity of the composites depended on the amounts of antioxidants and the duration of the thermal ageing treatment. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89:2316-2322, 2003.