By using X-ray photoelectron spectroscopy (XPS) and Fourier transform Raman (FT-Raman) spectroscopy, the worn specimens of polyetheretherketone (PEEK) tested under unlubricated sliding friction and wear conditions at a constant sliding speed were investigated in order to reveal mechanochemically and mechanophysically induced structural changes of polymer as well as wear mechanisms on a molecular scale. Chain scission was found on the worn surface layer. The results suggest that oxidation was the major mechanochemical reaction that followed the chain scission on the top surface. Evidence for chain branching or even crosslinking in bulk materials was also presented. Moreover, two-stage loading dependencies were found for both surface and subsurface in the bulk as revealed through wear rate measurement and wear debris analysis. It was proved that a thermo-activation of polymer segments may be responsible for the transition in the dependence of structure on load. The results of the present work also provide a method (based on spectral analyses) that can be used for studying micromechanisms accounting for shear deformation and failure.