Polypropylene (PP) fibres for geotextile applications were oxidized under two exposure conditions: 110 degrees C in air under atmospheric pressure and 80 degrees C under pure oxygen at 5 MPa pressure corresponding to two distinct ISO standards. Fibre ageing has been monitored by rheometric molar mass, WAXS and tensile measurements. The results put in evidence the existence of predominant chain scission, chemicryrallization and embrittlement, the latter occurring at a structural state (M-W=130 kg mol(-1), crystallinity ratio 80%) almost independent of exposure conditions. The relationships between all processes are discussed.