PMMA films of 60 μm thickness were bombarded by various ions at energies of 35–160 MeV. In this way, a nearly uniform ionization density could be achieved over the full width of the sample foil. Hence, the evaluation of the optical properties (refractive index and absorption) could be obtained unambiguously by standard optical techniques. With increasing deposition of ionization energy, an increase of the refractive index by a few percent could be observed. This increase can be interpreted as the consequence of splitting-off side chains; this causes a compaction of the material (increased density) and an increasing number of double bonds formed along the polymer skeletal chains.