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Oxidation is the dominant ageing process for polypropylene (PP) geotextiles (GTX) not exposed to mechanical stress or UV radiation. Resistance against oxidation depends strongly on temperature. In certain applications like in landfills geotextiles have to fulfil their functions for very long service lives at elevated temperature. In an extensive test program one nonwoven (GTX-N) and two woven geotextiles (GTX-W), all made of PP were subjected to thermo-oxidative ageing in high-pressure autoclave testing (HPAT) at temperatures between 75 °C and 90 °C under elevated oxygen pressures ranging from 200 kPa to 500 kPa. The progressive effect of oxidation was monitored by carrying out tensile tests and by noted changes in oxygen pressure in the autoclave during exposure. A decrease in oxygen pressure can be attributed to consumption of oxygen molecules and therefore oxidation of the polymer. Both – tensile properties and decrease in oxygen pressure – are used to determine times-until-failure. By varying the temperatures and the oxygen pressures in the autoclave, times-until-failure extrapolations to service conditions (e.g. 40 °C, 21 kPa oxygen pressure) were carried out resulting in realistic service lives. The resultant activation energy determined was between 84 kJ/mol and 93 kJ/mol. Additional oven ageing tests were carried out at 90 °C and 110 °C in order to compare both test methods. Depending on the stabilization of the tested GTX their lifetimes varied considerably under the anticipated elevated temperatures occurring in landfills.