Chitosan, a deacetylated derivative of chitin, is a biodegradable cationic polymer. Chitosan can be a potential substitute for aluminum salts and synthetic polyelectrolytes in water treatment because it can: (1) avoid the health effects from residual aluminum (III) and synthetic polymers; (2) produce biodegradable sludge; and (3) reuse the crab shell. Chitosan can be modified with various pretreatments including dissolution in acid solution and various deacetylation conditions to improve its coagulation efficiency. In this study, coagulations of synthetic and real water with the modified chitosan, mixed coagulant of chitosan and PACl at various ratios, and PACl were compared. The efficiency was evaluated by the factors of settling rate, floc diameter, and residual turbidity. Coagulation with chitosan alone required least dosage. Chitosan coagulants also produced larger floc with higher settling velocity. Satisfactory results were obtained from the mixed coagulant of chitosan/PACl=4:1. Although low pH decreased the optimal dosage, the flocs produced were smaller with slower settling rate. Increasing the rapid mixing strength improved the coagulation only when the applied dosage was less than the optimum dosage. Studies using other particles such as kaolinite and clay indicate that the properties of particles have a significant effect on the chitosan coagulation efficiency. © 1999 Elsevier Science B.V. All rights reserved.