Polymer films used as encapsulant materials in PV modules are responsible for the optical and mechanical connection of different layers of the module, electric isolation, protection of the solar cell against mechanical damage and work as a barrier against environmental impurities. The application range of PV modules is expected to be up to 25 years with only 20% reduction in the power output during that time. Changing environmental conditions as day-night-cycles, annual thermal changes as well as weather conditions like hail, rain, wind or dust cause high requests in the material properties and demand a sophisticated knowledge on the ageing behaviour of these polymers. In this thesis the focus is laid on the characterisation of the chemical ageing processes of the commercially most applied ethylene-vinyl acetate copolymer (EVA) and an alternaive polyethylene (PE) encapsulant. To simulate conditions found in PV modules the samples were artificially aged using damp heat exposure (85 °C/85 RH) and xenon weathering (90 W/m², 60°C, wet/dry cycles) for 2000 hours. The encapsulant materials were exposed as single films and incorporated in a laminate to observe the ageing processes in different microclimates at the polymer films. Infrared spectroscopy, Raman spectroscopy and differential scanning calorimetry measurements showed the formation of various photo-oxidation products, C=C double bonds, post crosslinking, fluorescence and changes of the polymer structure of the encapsulants. Ultraviolet/Visible/Near-infrared spectroscopy detected discolouration and changes of the reflectance properties of the polymers and with high performance liquid chromatography additives present in the polymers could be determined. It was shown that the exposure of the encapsulant materials as single films or incorporated in a laminate resulted in very different ageing behaviour due to the different microclimates at the polymer films.