Abstract For approximately 2 years the large full-scale municipal membrane bioreactor (MBR) in Kaarst/Germany has been monitored and sampled. In addition to operational data for the membrane and the bioreactor and an expanded set of water and sludge quality parameters, analytical data for dissolved and colloidal substances were collected and analysed. For the first time a complete dataset on the occurrence of extracellular polymeric substances (EPS) in a full-scale MBR and their relation to membrane fouling are reported. Additionally, several other full-scale MBRs in North Rhine-Westphalia (Germany) were investigated with a focus on extracellular polymeric substances (EPS) and their effect on membrane fouling. The analysis contributes to the understanding of permeability-reducing factors and fouling mechanisms in a full-scale MBR-installation. On the basis of a broad monitoring of activated sludge parameters both the evidence of temperature impacts on the formation of foulants, settleability and filterability of the sludge as well as on permeate flux were observed. Pronounced seasonal variations were detected with respect to soluble carbohydrates and other macromolecular compounds in the sludge supernatant. Especially the correlation between low temperature periods and the concentration of soluble carbohydrates is of importance, as it suggests adjustments in cleaning frequency during these periods. These variations can be related to conventional sludge parameters like capillary suction time (CST) and sludge volume index (SVI). A correlation to filtration parameters (filtration index Fl 15, total filtration resistance) was not observable. It could also be observed that DOC is an alternative to more complex and costly measurements of soluble EPS components to characterise sludge supernatant. Nevertheless, neither can be used as reliable predictive measures for filtration performance. At this point, a regular check of sludge parameters remains essential for sustainable operation of full-scale MBRs.