The constants of complexation of extracellular polymers (ECPs) extracted from sludges for three metals, Cu, Ni and Zn at pH 7, were determined using a polarographic method (stripping mercury dropping electrode mode (SMDE)). The curves obtained were exploited according to Chau's or Ruzic's method. The influence of the biochemical composition of ECP on the binding of Cu, Ni and Zn was investigated on six solutions of ECR Polarography in SMDE mode is a simple method to determine complexation constants of ECP extracted from activated sludges. For each, the number of sites of complexation increased in the following order: Zn much less than Ni < Cu. The low number of binding sites onto ECP for Zn did not allow the determination of a constant of complexation. For ECP studied, the constant of complexation was always higher for Cu than for Ni. A statistical study showed that the higher the content of proteins, humic acids and polysaccharides contained in ECP, the more they were able to bind high quantities of Cu. For Ni, the parameters of complexation were linked to the amount of uronic acid. These results suggest that carboxylic groups play a major role in Ni and Cu complexation by ECP at pH 7. (C) 2003 Elsevier Ltd. All rights reserved.