To provide insight into aluminum (Al) speciation in treated water and to investigate the factors that can affect it, specific Al fractions have been isolated and analyzed in finished water samples for the coagulation treatment of a low DOC and high alkalinity reservoir water in Ji'nan (China). The organic poly-diallyl dimethyl ammonium-chloride (PDMDAAC) was added into poly-aluminum chloride (PAC) to produce the dual-composite coagulant (PAC-PDMDAAC). The particulate and organic matter removal during coagulation process was also measured as supporting data. It was intended to examine the influence of hydroxyl to Al molar ratio and PDMDAAC to Al mass ratio on the coagulation performance as well as the content and speciation distribution of residual Al in treated water. Experimental results demonstrated that the coagulation effect and Al fractions were quite different for single PAC and composite PAC-PDMDAAC coagulation system. The additional PDMDAAC can improve impurity removal and decrease the residual total Al concentration of the effluent. Yet, the deterioration of DOC removal at higher dosages and the increment of organically bound Al fraction in treated water can also be noticed by PDMDAAC introduction. The prehydrolyzed degree (basicity) and Al to PDMDAAC mass ratio in composite PAC-PDMDAAC should be determined by virtue of water quality and a preferential aim for coagulation. Majority of the total Al in treated water was particulate Al. In the dissolved Al fraction, organically bound Al forms were present at a higher proportion. (C) 2012 Elsevier B.V. All rights reserved.