During coagulation process, preformed and in situ formed Al-13 polymer are the predominant aluminum hydrolysis species at the optimal pH condition for polyaluminum chloride (PACl) containing high content of Al-13 and AICl(3) (AC), respectively. The structure and morphology of the flocs generated by preformed and in situ formed Al-13 polymer were investigated from the viewpoint of the relationship between Al speciation and flocs surface structure. The results showed that at pH 6 the predominant composition of PACl and AC flocs were Al-13 polymer aggregate and amorphous Al hydroxide, respectively. The flocs generated by preformed Al-13 polymer had a better crystalline structure than those from in situ formed one, while they had smaller size and smoother surface structure than those formed by in situ formed Al-13 polymer. During weak acidic pH condition and at high dosage of Al coagulant adsorption onto precipitated Al hydroxides was the main pathway for kaolin particle removal. Preformed Al-13 polymer showed a stronger charge neutralization ability, while the flocs generated from in situ formed Al-13 polymer exhibited a higher adsorption capacity for kaolin particles. Al species distribution during coagulation process governs the floc character and coagulation efficiency of Al coagulant. The present study further conduces to the understanding of coagulation mechanism and puts forward a basis for coagulant selection during water treatment process. (C) 2012 Elsevier B.V. All rights reserved.