Extracellular polymeric substances (EPS) are one of the main components of activated sludge. EPS can be found in two forms, soluble or bound depending on their localisation and/or their role in microbial metabolism. In this study, soluble and bound EPS are operationally defined: soluble EPS, which can be extracted by centrifugation alone, and bound EPS in floc biomass which require additional treatments for extraction. The two kinds of EPS extracted were characterized by their organic fractions, their carbon, nitrogen and phosphor-us contents, their biochemical composition and their pKa and PEC (protonic exchange capacity) values. Organic carbon content of EPS underlined qualitative differences between soluble and bound EPS, which were better established by polysaccharide content. At pH 7, whatever the EPS considered, the analysis of all biosorption results showed an EPS affinity for metals in descending order: CU2+ > Pb2+ > Ni2+ >> Cd2+. The study of EPS biosorption properties showed different behaviour for soluble and bound EPS depending on the metal studied. For CU2+ (only for EPS A), Pb2+ and Ni2+, soluble EPS showed stronger biosorption properties than bound EPS. For Cd2+ and CU2+ (case of EPS 13), biosorption properties of the two kinds of EPS studied were close due to the weak affinity of Cd 2, for EPS and the different possible binding mechanisms implicated by the speciation of CU2+, at pH studied. At pH 7, due to the PEC value and pK(a), the numbers of binding dissociated sites, could be assumed greater for soluble EPS than for bound EPS and could explain in part the biosorption results obtained. We can guess that soluble EPS play the role of a protective barrier against toxic metals for the microorganisms in the activated sludge flocs. (c) 2005 Elsevier Ltd. All rights reserved.