The relative importance of three different Al species, Al-a (monomeric species, instantaneous reacted species), Al-b (medium polymer species, reacted less than 120 min), and Al-c (colloidal or solid species, no reaction), defined by timed complexation reaction rate measured by using ferron reagent in polyaluminum chloride (PACl) was investigated in terms of DOC (dissolved organic carbon), UV254, and turbidity removal efficiencies. Micro-polluted, typical North China, source waters were used to conduct the experiments. The results show that DOC removal is correlated well to the content of Al-b. Removal of UV254 is determined by the content of Al-b and Al-c, particularly Al-c. Turbidity removal is primarily related to the content of Al-c; however, Al-b could destabilize particles efficiently, and the flocs formed by Al-b axe not as large as those formed by Al-c, which affected the settling efficiency. Unlike the preformed Al-b, the in situ formed Al-b could remove turbidity more efficiently since Al-c is the dominant final species formed during coagulation. Al-a shows a strong ability to react with some unsatisfied coordinate bonds of organic matter to facilitate particle and DOC removal. The distinct coagulation feature of Al-a, Al-b, and Al-c can be applied to develop tailor-made PACl (with the correct distribution of Al species) to match the characteristics of raw water for optimized coagulation. Crown Copyright (C) 2007 Published by Elsevier Inc. All rights reserved.