The energy loss function of ion beam irradiated polyimide has been deduced from optical and specular reflection in the range 2.5-6.5 eV. The appearance of a peak loss at 3 eV is the signature of the development of a sp 2 component responsible for optical gap narrowing and electronic conductivity. The different contributions to the photon energy loss function allowed the decomposition of the REELS (Reflection Energy Loss Spectroscopy) spectrum providing confirmation of a 3 eV component connected to the production of the conducting phase at high irradiation energy (170 keV Ne +). The formation of this band is partially inhibited at low energy irradiation (50 keV Ne +) as the proportion of nuclear transfers becomes significant. Rutherford Backscattering Spectrometry (RBS) shows that the diffusion of molecules can also be controlled by the irradiation modifications. Diffusion enhancement and gettering by the radiation damage is evidenced at low irradiation fluence. These modifications of the electronic and molecular transport properties are interpreted in relation with the irradiation induced nanostructure.