A stack of polyimide (PI) thin foils was irradiated with 900 MeV 40Ar 15+ ions up to a fluence of 8 × 10 11 ions/cm 2. Subsequently, the electrical conductivity and UV–VIS absorption spectra of specimens were measured for the selected foils from the irradiated stack. Two well defined characteristic absorption peaks at wavelengths 482 and 501 nm appeared after irradiation and their absorption peak intensities progressively increased with increasing electronic energy loss in the substrate. Over a range of deposited electronic energy density from 8 to 23 eV/nm 3, the depth profile of the chemical yields closely pursued the electronic energy transfer profile. The integrated optical absorption bands at 482 and 501 nm were used to evaluate the normalized damage fraction, the damage production cross-section and the track radii in the irradiated PI matrix as a function of deposited electronic energy density. The measured enhanced electrical conductivity substantiated by the optical band gap values was attributed to the formation of condensed polycyclic aromatic ring compounds and clusters with greater degree of conjugation in the multiple overlapping ion track in the PI matrix. The critical energy loss rate (track etch threshold) for 40Ar ion in PI was obtained from the optical absorbance and electrical conductivity measurements.