Fast heavy ions produce stable defects in most organic polymers. In this work poly(ethyleneterephthalate) was irradiated with Xe ions at an energy of about 11 MeV/u and an areal dose of about 2 × 1010cm−2. The tracks were characterized by soaking fluorescence tracers into the tracks using dimethylformamide as solvent. By analysing the band positions of solvatochromic dyes the local polarity can be obtained. A result of this characterization is that the dielectric number of the tracks is 10% decreased compared to the virgin polymer. By using the high polar solvent dimethylformamide as soaking agent it could be shown that there is no accomodation of this solvent in the tracks.