Abstract Polyethylene terephtalat (PET) foils were irradiated by 246 MeV Kr and 595 MeV Xe ions and the irradiated samples were studied by wide angle X-ray diffraction. A simple exponential law describes the variation of the integral X-ray intensity I int with the fluence. By applying the Poisson law 12.3 nm and 16 nm were deduced for the mean diameters of tracks induced by Kr and Xe ions, respectively. We did not observe considerable beam heating effect. The analysis of the irradiation experiments on polyvinylidene fluoride and on PET foils shows that the predictions of the thermal spike model of Szenes are in quantitative agreement with the experimental results.