Very thin (3 μm) polyethylene terephthalate (PET) foils were irradiated with 150 keV Na+ ions with the flu- ences in the range from 1 ×1014 cm−2 up to 1 ×1016 cm−2. Modification of chemical structure of the implanted polymer was studied with the Fourier transform infrared and Raman spectroscopies. Destruction of numerous chemical bonds as well as formation of carbon clusters made of sp2 hybridised C atoms and conducting cluster networks were demonstrated. The increasing presence of chain structures in the graphite-like carbonised layer is pointed out as the G band shift and decrease of D band could be seen as irradiation fluence rises. The large increase of the modified sample absorbance with the implantation fluence in the UV-VIS region was observed. The decrease of optical band-gap energy from 3.95 eV for the pristine sample down to 0.7 eV for the most heavily treated sample was estimated using the Tauc plot. Reduction of bulk resistivity by more than 8 orders of magnitude is shown in the case of the sample implanted with the fluence 1 ×1016 cm−2. The sheet resistivity of the sample was also reduced by 5 orders of magnitude. The effect was observed on both sides of the sample, probably as the result of chemical transformations due to local increase of the polymer foil temperature. The moderate increase (15–200%) of the dielectric constant is observed in the frequency range up to 1 MHz and the change rises with the implantation fluence.