The background of the presented work, were obervations on exterior parts of the BMW group. They tended to discolour unexpectedly although prior extensive qualification tests had been executed. The aim was to get in-depth understanding for impact factors and correlation effects during discolouration of black PMMA compounds. This should allow an optimised qualification process in the future. The investigated materials have been two PMMA compounds from the Romira GmbH named ROTEC® U470 and U470/07 as well as a compoundation based on the first two types. Furthermore a Plexiglas®-Type named 8N df23 of the company Evonik Röhm GmbH and a PC copolymer of the SABIC GmbH & Co. KG called Lexan have been investigated. To investigate the influence of water content, additives and irradiation time during artificial ageing, extensive statistical experiments were carried out. During the xenon tests the specimen were humidified with splash water as well as in a water bath. The aging process was determined with physical and chemical analysis. The composition and concentration of the additives were measured in a high pressure liquid chromatography. Ageing of the compounds was analyzed by infrared spectroscopy. With Karl-Fischer titration, the increase of water content during ageing was measured. In order to find out if elastomers were blended into the thermoplastic, differential scanning calorimetriy was carried out. The change in the molar mass distribution was measured with a gel permeation chromatography on differently discoloured areas on a part from the field. As a result of this work, a special irradiation method under water called SunFlood was validated as an appropriate ageing test in the qualification process of thermoplastics. Discolourations due to hydrolysis and those due to water absorption can quickly be detected. Moreover a deep going understanding for chemical reactions and interaction effects during irradiation is created. Based on those new findings, future compounds can be specifically designed to meet the demands of a good colour stability.