Membrane (ultra)filtration of activated sludge from membrane bioreactors (MBR) is usually performed with low fluxes. This is done to prevent membrane fouling and system failure. However, even when operated with low fluxes, filterability of the biomass can fluctuate in time considerably. Among the many causes that are mentioned to explain this phenomenon, changes in influent quality and quantity seem most probable. Until now, only little work was done to quantify effects of transient conditions in feeding pattern of an MBR. This paper describes experiments in which 25 L samples of the biomass from an MBR system were fed with an easily biodegradable carbon source. The filterability of the biomass was monitored with standardised filtration characterisation measurements. Along with filtration characterisation the water phase of the activated sludge was analysed for extracellular polymeric substances (EPS). Due to the addition of acetate the filterability of the biomass deteriorated, whereas the EPS concentration in the water phase increased. Activated sludges from different MBR-pilots react differently in terms of filterability development.