Wavelength division multiplexing (WDM) is essential for further enhancement of the transmission capacities of optical telecommunication systems. Key devices in WDM networks are multiplexing/demultiplexing components, which enable the combination/separation of several carrier waves with different wavelengths for the purpose of simultaneous transmission through one optical fibre. These components can be realized using Bragg diffraction from volume holographic gratings. Especially reflection holograms provide a pronounced wavelength selectivity which makes them attractive for free-space WDM applications. Holograms can be stored permanently in photorefractive lithium niobate crystals by the method of Thermal Fixing. Heating of the crystal during or after the recording process and subsequent development by homogeneous illumination at room temperature create nonvolatile holograms. The recording and development processes of Thermal Fixing in iron- and copper-doped lithium niobate crystals were investigated. Macroscopic Gaussian-shaped intensity patterns were used to analyse the origin of the fixing mechanism. Spatially resolved absorption measurements were performed to determine the concentration profiles of electron traps (Fe II/III) and protons. Results of computer simulations were compared with experimental results, which showed that protons can be found to work as compensators during hologram recording at temperatures around 180 degree C. Nevertheless thermal fixing without protons was possible, another compensation mechanism stood in. The obtained refractive-index changes were due to the electro-optic effect, other contributions could be neglected. With this detailed knowledge about thermal fixing, a two-channel demultiplexing unit was built by superposition of two thermally fixed reflection holograms in an iron-doped lithium niobate crystal. For this purpose a special two-beam interference setup with precisely adjustable writing angles was arranged in a vacuum chamber to eliminate thermally induced phase disturbances of the holographic recording procedure. Continuous development of the holograms by incoherent light was necessary. In the dark, the enhanced dark conductivity of the crystal used gave rise to a hologram degradation within about one day. Large diffraction efficiencies were attained (intensity losses between 2.3 and 5.2 dB only) uilizing crystals with high-quality polished surfaces. The crosstalk supression of the realized demultiplexer was > 25 dB, which is comparable with the performance of other multiplexing techniques like fibre Bragg gratings or arrayed-waveguide gratings. The low polarization dependence of the demultiplexer can be improved by superposition of two holograms for each channel.