Interactions between sulfur and metals were studied in aerobic and anaerobic biofilms grown on domestic waste water at 15°C. The dominant metals in the waste water were iron, zinc and copper, which were present in average concentrations of 0.5mg/l, 0.6mg/l and 0.1m/l, respectively. Copper and zinc were found to accumulate in the anaerobic biofilm owing to precipitation of metal sulfides. Iron supplementation (approximately 5.5mg Fe/l) to the anaerobic waste water reduced the zinc and copper precipitation due to sulfide precipitation with iron. However, even at these elevated iron concentrations in the waste water, sulfide precipitation in the biofilm was controlled largely by zinc and copper. In the aerobic biofilm, addition of iron resulted in accumulation of iron and phosphate, probably owing to precipitation of iron phosphates and iron (oxy)hydroxides. The potential importance of extracellular polymeric substances (EPS) on metal sorption in sewer biofilms was studied. EPS consisted mainly of proteins (13–260mg/(g volatile solids)) and to a lesser extent of carbohydrates (8–26mg/(g volatile solids)). The EPS composition remained relatively constant during experimental runs, but differed significantly between them. No relationships between the metal content of the biofilm and the amount of extracted EPS were found, which suggests that EPS did not play a major role in the metal accumulation.