Iodine sorption in un-irradiated and ion-irradiated polycarbonate (PC) has been investigated using positron lifetime spectroscopy. The decrease in positron lifetime parameters of ion-irradiated sample is attributed to the free volume modifications, and the formation of more stable free radicals on cross-linking. The difference UV absorption spectrum shows a strong absorption peak at 384 nm which is due to 2,2'-dihydroxybenzophenone; this depicts the possibility of photo stabilisation of polycarbonate on ion-irradiation. A comparative study of sorption process is explained in terms of iodine diffusion in free volume holes in un-irradiated and ion-irradiated polycarbonates. The experimental results on iodine diffusion shows an early saturation of positron parameters in irradiated PC, which is due to an increase in the rate of diffusion as small number of bigger size free volume holes are formed on irradiation. The diffusion process follows Fick's law and, an exponential type of correlation has been observed between fractional free volume and diffusion coefficient which indicates that Fujita's free volume theory is valid before and after ion-irradiation.