The three-dimensional excitation emission matrix (EEM) fluorescence spectroscopy was employed to investigate the interaction of extracellular polymeric substances (EPS) from natural biofilm with Hg(II). The EEM spectra demonstrated that EPS with molecular weight over 14 kDa had two protein-like fluorescence peaks. The fluorescence intensity at both peaks was strongly dependent on the solution pH in the absence and presence of Hg(II), with the maximal fluorescence intensity at neutral pH. Fluorescence of both protein-like peaks was significantly quenched by Hg(II). The values of conditional stability constants (log K-a = 3.28-4.48) derived from modified Stern-Volmer equation are approximate to those for humic substances and dissolved organic matter (DOM). indicating that fluorescent components in EPS have strong binding capacity for Hg(II). Our findings suggest that EPS from biofilm is a class of important organic ligands for complexation with Hg(II) and may significantly affect the chemical forms, mobility, bioavailability and ecotoxicity of heavy metals in the aquatic environment. (C) 2009 Elsevier B.V. All rights reserved.