The formation of carbonaceous clusters in ion-irradiated polymer films was investigated extensively. Information about these clusters may be obtained with ultraviolet-visible (UV-vis) spectroscopy. The optical band gap (E g ), calculated from the absorption edge of the UV spectra of these polymers, can be correlated to the number of carbon atoms (N) in a cluster with the modified Tauc equation. The structure of the cluster is also related to E g ; for example, a six-membered-benzene-ring-type structure has an E g of Ϸ5.3 eV, whereas a buckminsterfullerene-type structure has an E g of Ϸ4.9 eV. These clusters are responsible for the electrical conductivity in these films. In this work, polycarbonate films (20 m thick) were irradiated with 45-MeV Li ions at fluences of 1 ϫ 10 12 to 1 ϫ 10 13 cm Ϫ2 and were characterized with UV-vis spectroscopy and impedance measurements. The E g values, calculated from the absorption edge in the 280 -315-nm region with the Tauc relation, varied from 4.39 to 4.35 eV for the pristine and various irradiated samples, respectively. The cluster size showed a range of 60 -62 carbon atoms per cluster. The sheet conductivity ( dc ) and loss (tan ␦) values of 10 Ϫ16 ⍀ Ϫ1 cm Ϫ1 and 10 Ϫ3 for the pristine sample changed to 10 Ϫ15 ⍀ Ϫ1 cm Ϫ1 and 10 Ϫ2 , respectively, for the irradiated samples. This increase in the values of dc and tan ␦ may be correlated to the increase in the size of the carbonaceous clusters. This study provides insight into the mechanism of electrical conductivity in irradiated polymers.