Extracellular polymeric substances (EPS) are a major component of biofilms that act as a gel-like matrix, binding the cells together to form their three-dimensional structure. The effects of ceria nanoparticles (CeO2 NPs) on the production and physicochemical characteristics of EPS in biofilms in a sequencing batch biofilm reactor were investigated. Total EPS production, including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), increased by 35.41% compared to in control tests without CeO2 NPs. Protein production increased by 47.02% (LB-EPS) and 58.83% (TB-EPS) after 50 mg/L CeO2 NP exposure. Three-dimensional excitation-emission fluorescence spectra revealed that tyrosine (LB-EPS) and aromatic (TB-EPS) protein-like substances formed after CeO2 NP exposure. Fourier transform infrared spectroscopy results indicated the susceptibility of -OH and -NH2 in EPS hydroxyl and amine groups to CeO2 NPs. Exposure to 50 mg/L CeO2 NPs reduced the flocculating capacity of LB-EPS (51.78%) and TB-EPS (17.14%), consistent with the decreased zeta potential. (C) 2015 Elsevier Ltd. All rights reserved.