In recent years, membrane bioreactors (MBR) have become an important focus in municipal and industrial wastewater treatment. MBR offers many advantages compared to the conventional activated sludge (CAS) processes, for example, the first one allows working at high concentration of mixed liquid suspended solids (MLSS), produces high rates of organic matter and efficiency in removal of nutrients, and presents a small footprint. Nevertheless, the major problem involved in operating membrane bioreactors (MBR) is membrane fouling. This is caused by the presence of wastewater constituents such as colloids. mineral particles, ionic components, microorganisms and it is basically attributed to extracellular polymeric substances (EPS). These are high-molecular weight compounds secreted by microorganisms, which are mostly composed of proteins and polysaccharides. EPS can appear as supernatant and bound EPS, and the latter have to be extracted from flocs. For this reason, some extraction methods have been developed. An exhaustive study of the function, behaviour and characteristics of EPS is essential to improve MBR operation. In this paper, a comparison among four extracts of EPS obtained from different extraction methods was done, in terms of their molecular weight (MW) distribution and average MW using High-Pressure Size Exclusion Chromatography (HPSEC). Results showed that those extracted using chemical techniques, do not affect the MW distribution fingerprints of EPS, or their average MW. Nevertheless, physical extractions showed different behaviour in EPS fingerprints. (C) 2010 Elsevier B.V. All rights reserved.