The influence of advanced oxidation processes (AOP) on natural organic matter (NOM) is theme of the project "Advanced Oxidation Processes in Drinking Water Treatment - Minimisation of Disinfectants and Reduction of Bacterial Regrowth". The project is sponsored by the Federal Ministry for Education and Research (BMBF) and is a co-operation of three water supply companies and three research centres. Parts of the NOM are reason for the production of chlorine-organic disinfection by-products (DBPs), which are possibly toxic. Also, bacterial regrowth in supply net is caused by NOM-substances. The production of hygienic safe water by changing the properties of NOM with oxidation processes is basic aim of the project. The Engler-Bunte-institute is comparing the changes of the NOM caused after the treatment with three different kind of AOPs (ozone/UV-, hydrogen-peroxide/UV- and ozone/hydrogen-peroxide-process) and after ozonation. To characterise the NOM gel chromatography with DOC-detection is used. The results of gel chromatography were correlated with the results of DBP- and bacterial regrowth experiments. First results show that AOP, which use ozone, are more effective in reducing DBPs than the hydrogen-peroxide/UV process. The bacterial growth after AOP treatment is lower than after ozonation.