In this study new photoreactive polymers, based on polynorbornene, were investigated with the aim to create large refractive index changes under two-photon conditions. In order to obtain flexible materials suitable for 3D patterning, strategies for crossliniking of the polynorbornene backbone were assessed. Six functionalized polynorbornenes bearing aryl ester groups in the side chain were synthesised and characterised. Under UV irradiation, a photo-Fries reaction and/ or a photo-decarboxylation (extrusion of CO2) proceeded as evidenced by FTIR and UV spectroscopy. Ellipsometry showed that the refractive index of the polymers increased significantly as a result of the photoreaction. Experiments aiming at two-photon patterning of these polymers using a Ti:sapphire laser (600 800 nm) succeeded to write 3D waveguides. The waveguide structures were revealed by phasecontrast microscopy. Moreover, the thiol-ene reaction was employed to crosslink the polynorbornene-backbone using thermal and photochemical initiators. The large increase in the refractive index and the possibility to create 3D patterns under two-photon conditions makes these polymers good candidates for optical applications such as holographic recording and fabrication of optical elements, e.g. waveguides.