The major operational problem associated with membrane bioreactors (MBR) is membrane fouling, for which extracellular polymeric substances (EPS) are primarily responsible. In this work both the soluble and bound EPS (i.e. SMP and EPS) produced in an MBR system operating under sludge retention times (SRT) of 10, 15, 20 and 33 days were fractionized by means of membranes having variable molecular weight cutoffs (300 kDa, 100 kDa, 10 kDa & 1 kDa). The results show that increasing the SRT leads to a reduction of SMP and EPS and that these reductions are more pronounced for the SRTs in the range 10-20 days. This reduction is more significant for carbohydrates than for proteins. The decrease of SMP and EPS with increasing SRT from 10 to 20 days led to a significant decrease of the level of fouling. The further increase of SRT to 33 days did not significantly impact on the level of fouling as the SMP and EPS concentrations did not change much. Under the examined operating conditions, EPS were found to be composed mainly of large macromolecules having a size of 0.45 mu m-300 kDa and to a lower extent of very small molecules (< 1 kDa) that are not easily decomposed by the biomass activity. The majority of SMP is composed of very small molecules (< 1 kDa), while some macromolecules in the range of 0.45 mu m-300 kDa are present. Consequently, both EPS and SMP were found to have a bimodal character. (C) 2009 Elsevier Ltd. All rights reserved.