Influent chemical oxygen demand/nitrogen (COD/N) ratio is used to control fouling in membrane bioreactor (MBR) systems. However, COD/N also affects the physicochemical and biological properties of MBR biomass. The current study examined the relationship between COD/N ratio in feed wastewater and extracellular polymeric substances (EPS) production in MBRs. Two identical submerged MBRs with different COD/N ratios of 10:1 and 5:1 were operated in parallel. The cation concentration and floc-size of the sludge were measured. The composition and characteristics of bound EPS and soluble microbial products (SMP) under each COD/N ratio were also examined. Batch tests were conducted in 1000 mL bottles to study the process of the release of foulants from the sludge when 1 g of (NH4+)/L was added. Results showed that the influent COD/N ratio could change the physicochemical properties of EPS and SMP. Moreover, excessive NH4+ in the supernatant could facilitate the role of NH4+ as a monovalent cation, the replacement of the polyvalent cation in bound EPS, and even the extraction of EPS components from the surface of the sludge to form new SMP. (C) 2011 Elsevier B.V. All rights reserved.