A 3 MeV He + ion beam was used both for degradation and characterization of Polyethylene Terephtalate (PET) film, the thickness was 2.5 and 10 µm. To widen the possibilities of elemental analysis in situ we used both Rutherford Backscattering Spectrometry (RBS), which transforms to Nuclear Backscattering Spectrometry (NBS) at the energy E α = = 3 MeV and elastic recoil detection (ERD) methods at the same time, combining them with Particle Induced X-ray Emission (PIXE). RBS (NBS) was used for carbon and oxygen analysis, ERD for hydrogen analysis and PIXE for impurity heavy ion detection. PET films was placed on rotating frame to minimize heating during irradiation. Also it allowed to enlarge the irradiated surface to control fluence range from 10 10 to 3∙10 15 cm -2. Detection angle were 160° for RBS and 30° for ERD. PIXE detector was placed in vacuum chamber near the sample. Figures 1 and 2 shows the changing of NBS spectra after irradiation of PET sample and decreasing of oxygen concentration with the irradiation fluence. After He + ion irradiation we notice changing of the yield of backscattered particles from irradiated sample. It shows the reduced concentration of carbon and oxygen. At the fluence of 5∙10 15 cm -2 the concentration of oxygen approaches to 2 at. % (instead of 18 at. % in unirradiated sample), carbon and hydrogen show less depletion and have concentration of 18 at. % instead of 24 at. % for hydrogen and 38 at. % instead of 45 at. % for carbon. Volatile degradation products escape from irradiated surface most in form of H 2, O 2, CO and CO 2 [1, 2]. Element losses have different rates and different stabilization levels. Ion beam irradiation also lead to concentration of heavy ion impurity because it doesn’t leave the sample as C, O, H. The element forming this impurity (Sb) was determined using PIXE. Final losses are about 90% of oxygen, 50 % of hydrogen and up to 17 % of carbon, comparing with 85 % O, 45 % H and 15 % [3].