Ion beam bombardment induced effects on the crystalline and chemical structures, as well as in the thermal, optical, and electrical properties of PET (Mylar) were studied. The induced modifications were followed by Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-VIS), X-ray diffractometry, differential scanning calorimetry (DSC), and electrical resistance measurements. The melting temperature and the crystalline fraction started to decrease at fluences where the chemical degradation was not significant. The hydrogen and oxygen losses led preferentially to new carbon-carbon bonds within the polymer chains and gradually increased the aliphatic and aromatic conjugation. Simultaneously, a decrease in the optical gap as function of the ion fluencc is observed. For 40Ar 2+-bombarded samples the optical gap saturates around 0.7 eV lor f'luences of the order of 10 15 cm -2, At those fluences the electrical resistivity is relatively high (τ ≫ 10 6 Ω cm) , but for higher fluences it decreases by several orders of magnitude before saturation. The cross sections for the amorphisation, for the optical gap change and for the ester group bond reorganization processes were extracted.