The thermal oxidation of unstabilized polypropylene films has been studied at 70, 90, 110 and 130degreesC in air by IR, SEC, rheometry and tensile testing. Embrittlement is observed when the weight average molar mass reaches 210 +/- 10 kg mol(-1). Polydispersity data are consistent with a homogeneous random chain scission process. This especially high sensitivity of fracture properties to chain scission is assumed to be specific of semi-crystalline polymers having their amorphous phase in rubbery state. (C) 2004 Elsevier Ltd. All rights reserved.