Polymethylmethacrylate (PMMA) has been irradiated by UV-laser light with different wavelengths (193, 248, 308 nm) in order to modify photolytically its refractive index. The photoinduced chemical reactions at the polymer surface have been investigated by QMS, XPS and FTIR and the mechanism of the structure modification has been clarified. It was shown that depending on the experimental conditions, an increase of the refractive index in the modified zones could be achieved. The controllable local increase of the refractive index was used to prepare passive integrated-optical waveguides in PMMA chips.