The influences of C/N ratio on the extracellular polymeric substances (EPS) and physicochemical properties of the activated sludge flocs were investigated using laboratory-scale sequencing batch reactors (SBRs). Flocs sizes decreased when C/N ratio increased from 20 to 100 and decreased from 20 to 4. The amount of total EPS, TB-EPS, and the carbohydrate and protein contents in TB-EPS were independent of the C/N ratio. In LB-EPS, the protein content increased and the carbohydrate content decreased at decreased C/N ratio, whereas the protein content decreased and the carbohydrate content increased at increased C/N ratio. Effluent suspended solids (ESS) content, turbidity, sludge volume index (SVI), capillary suction time (CST), and specific resistance to filtration (SRF) increased when the C/N ratio decreased, indicating poor flocculation, settleability and dewaterability of the flocs. However, when the C/N ratio increased, only ESS content, SVI and CST value increased. These properties of the flocs were deteriorated greatly under decreased C/N ratio as compared to increased C/N ratio. The characteristics of the flocs could be recovered when C/N ratio returned to the original value. Only the content of protein in LB-EPS was positively correlated with the flocculation, settleability and dewaterability of the flocs. (C) 2011 Elsevier B.V. All rights reserved.