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Embrittlement processes occurring during thermal oxidation are investigated for stabilized and unstabilized polyamide 11 samples differing by their thicknesses and initial molar masses. Tensile tests were carried out in the temperature range between room temperature and 110 C in order to investigate the influence of mechanical testing temperature on the embrittlement coordinates. In the same time, molar mass and crystalline morphology are monitored by size exclusion chromatography (SEC) and DSC/SAXS measurements respectively. The experimental results point out the existence of a critical molar mass for ductile-brittle transition M 0 c about 10 kg mol À1 , independent of sample initial molar mass or stabilization, but depending on tensile testing temperature. However, even if oxidation chain scissions are shown to be clearly responsible for the loss of mechanical properties at failure, the structure-property relationships governing ductile-brittle transition require a mixed criterion involving molar mass and crystalline morphology, especially the interlamellar distance. For this purpose, specific molar mass e crystalline morphology relationships are investigated.