This experimental study investigated the modification of optical, structural and morphological properties of Makrofol-KG and Polyethylene Terephtalate (PET) by irradiation of 120 MeV Ni(+9) ions at fluences ranging from 1 x 10(10) to 1 x 10(12) ion/cm(2). Optical and structural properties were studied by UV-visible spectroscopy and X-ray diffraction (XRD) method, respectively. XRD studies of pristine and irradiated polymers revealed that crystallinity decreased after irradiation with swift heavy ions. The UV-visible spectrum displayed a shift in absorbance edge toward higher wavelength at higher fluence, which can be correlated with transition involved in polymers. Fourier transform infrared spectroscopy spectra were analyzed to investigate the changes in the chemical/structural properties of these polymers. The Makrofol-KG is found to exhibit more pronounced modification compared to PET after irradiation by swift heavy ions. The surface morphology studied by atomic force microscopy showed that the roughness of polymers surface increased with increasing fluence. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121:3014-3019, 2011