Although pre-chlorination has been widely applied to enhance the removal of algae, the mechanisms involved in the process have not been well illustrated. This study used Microcystis aeruginosa to investigate the effects of chlorination on the algae cells (i.e., surface charge, cell integrity, and release of intracellular organic matter (IOM)), and the subsequent influence on algae removal by alum coagulation. Though intracellular K+ and IOM released after the exposure of M. aeruginosa to chlorine, no considerable change in the morphology was observed for the majority of algae cells. This indicated the significance of dissolved organic matter (DOM) in the removal of algae, which consisted of released IOM and natural organic matter (NOM). Chlorine dose had a substantial impact on the concentration and characteristics of DOM, and accordingly on the optimum dosage range of alum for effective coagulation of algae cells. High molecular weight (MW) DOM aided the coagulation by favoring the formation of larger flocs and decreased the optimum alum doses to achieve the maximum removal of DOM. Released IOM also had high ratios of protein/IOM to form protein-alum complexes, which increased coagulant demand and inhibited the cross linking and clustering of alum-hydroxide polymers to disturb coagulation. (C) 2011 Elsevier B.V. All rights reserved.