In this work, the thermo-oxidative behavior of PEKK films (~50 µm thick) in solid state were investigated. Since the mechanical properties of PEKK samples, especially the strain at break, are clearly affected after long ageing time, the modifications of the PEKK structure have been monitored at the molecular and the macromolecular scales using different techniques such as gel permeation chromatography, X-Ray scattering, differential scanning calorimetry, and dynamic mechanical analysis. If the embrittlement of the PEKK specimens seems to appear due to a predominant branching/cross-linking phenomenon occurring at the macromolecular scale, crystalline morphology is also slightly modified due to a chain scission mechanism occurring simultaneously.