The resistance of high density polyethylene to thermal oxidation has been improved by addition of highly dispersed particles of Fe-FeO. Specimens were exposed to thermal aging in air at 80 degrees C, and their mechanical behaviour under tensile stress was then investigated. It was shown that Fe-FeO acted as a stabiliser for high density polyethylene, improving its thermal stability and mechanical properties. The factors responsible for these improvements were an increase in the amount of interphase between the crystalline and amorphous phases, decreased crystallite size, and increased network entanglement density. PRC/1380 (C) 2000 IoM Communications Ltd.