To be able to implement light-sensitive properties of ionimplanted PMMA in optical devices, it is first necessary to investigate the optical stability of the implanted layers. For this purpose, PMMA samples were implanted with nitrogen using doses from 7×1013/cm 2 to 5×1015/cm2 as well as energies from 100 keV to 380 keV. Subsequently these samples were exposed to dry media such as H2, 02, CO2, N2, Ar as well as to dry and humid atmosphere. In a further investigation, the optical stability was examined under the influence of different wavelength ranges. The changes resulting in these layers were ascertained over various periods (maximum of 5 years) by ellipsometric measurement of the refractive indices and, in parallel, the chemical modification of the surface was examined by means of XPS. If any changes were caused by the media, they consisted basically in a decrease of the index values and of the original concentrations on CH— and COH—groups generated by the bombardment— i.e. in a recombination of molecular chains. In correlation to the chemical modification of the surface, the refractive index partly changed drastically.