Polyamide 6 films were immersed in two ageing environments inducing either only oxidation or only hydrolysis of the polymer for up to two years. Ageing temperatures ranged from 80°C to 140°C. Samples were characterized periodically in terms of both chemical structure at the macromolecular scale, using SEC, DSC, SASX and WAXS, and mechanical behaviour through tensile tests. Both degradation mechanisms lead to chain scission within the polymer, an increase in crystallinity ratio, a decrease in the amorphous layer thickness and an embrittlement of the polymer. First a decrease in the strain at break is observed while the maximal stress remains unchanged. Then a drop in maximal stress is identified. Using these experimental results, both the origin of the embrittlement and the factors governing embrittlement are discussed. The decrease in strain at break is attributed for the first time in polyamide to the decrease in concentration of tie molecules determined through a theoretical approach. The loss in entanglements is associated with the drop in maximal stress. Furthermore, it is shown that the crystallinity ratio does not govern the embrittlement of polyamide. However, both the molar mass and the amorphous layer thickness are faithful indicators of this embrittlement whatever the degradation mechanism.