The ion beam-induced modification of the electrical conductivity and the related structural features have been studied for thin films of poly-iso-quinazolindione (PIQ). The films, deposited onto specially designed test patterns, were irradiated by using 600 keV Ar+ ions in the fluence range between 1 × 1014 to 1 × 1015 ions/cm2. The beam-induced chemical and structural modifications have been investigated by using X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy, while the modification of the electrical properties was followed by performing a complete set of I/V measurements. In particular, we obtained the evidence of the occurrence of a true semiconducting state for samples irradiated at fluence around 6 × 1014 ions/cm2, which exhibited the characteristic I/V features of a Schottky diode. Samples irradiated at higher fluence showed a good conductivity, with a saturation value of 4 × 102 Ω cm. The XPS data demonstrate that the modification of the electrical properties is due to the progressive formation with increasing ion fluence of a dense amorphous carbon network, while Raman data suggest the existence of different structural features, respectively for the semiconducting and the conducting phases.