Polymers play an increasing role in technical applications. Presently the polymer polymethylmethacrylate (PMMA) has been found to be a suitable materials for the fabrication of optical devices, where the refractive index can be altered considerably by ion bombardment. Drastic increases of the refractive index have been observed after ion implantation at doses of the order of 1014/cm2. The dependence of the refractive index on ion energy, ion dose and ion species was investigated for H+, He+, N+, N2+, C+, O+, F+, Si+, Ar+, Zn+, Kr+, and Xe+at energies ranging from 100 keV to 350 keV for ion fluences between 1013/cm2and 4 × 1014/cm2. The radiation sensitivity and the attainable changes of the refractive index in PMMA were found to exceed drastically any previously known value. Therefore, this material may be judged to become suitable for many kinds of future micro-optical applications.