At the Ruhleben wastewater treatment plant (WWTP) two membrane bioreactor (MBR) pilot plants have been operated since September 2001 by Veolia Water and Berliner Wasserbetriebe. The primary aim of the piloting is the investigation of biological phosphorus removal in conjunction with nitrification/denitrification in MBRs for later use in remote areas and small scale applications (WWTP serving a few thousand inhabitants) [Gnirss et al 2003a]. Both plants are fed with the same raw wastewater as it is treated in the conventional wastewater treatment plant. Instead of the mechanical treatment of the conventional plant, the raw wastewater passes a 1 mm punch hole screen prior to the biological treatment in the two MBR pilot plants. The two pilot plants are operated under parallel operating conditions (same raw wastewater, same sludge age and sludge concentration , etc.), but there are two different biological process configurations: pre-denitrification and postdenitrification without addition of a carbon source. Over the first year of operation, it has been observed that the unit with post-denitrification exhibited more rapid membrane fouling than the one with pre-denitrification. Preliminary LC-OCD (liquid chromatography-organic carbon detection) measurements carried out with the permeate compared to paper filtered sludge showed differences between the two units regarding the concentration of colloids and large macromolecules (as measured in the polysaccharide peak). Hence, an assessment and investigation of the fouling behaviour of the two MBR pilot plants was commenced. The results are presented in this report.