Effect of pH on the aluminum chloride hydrolysis at low concentration was investigated in detail by electrospray ionization (ESI) mass spectrometry. In particular, formation and decomposition processes of polymeric aluminum species were discussed. When Coagulant AlCl3 was diluted to normal coagulant dose (1.5 x 10(-4) mol/L), hydrolysis occurred immediately. Monomeric and dimeric aluminum species were the main products at pH 4.0. With pH increasing, hydrolysis and polymerization processes were accelerated. Monomeric and dimeric aluminum species hydrolyzed and polymerized into small polymeric aluminum species (Al-3-Al-5 species) at pH 4.8. Through aggregation and self-assembly, the small polymeric aluminum species polymerized into median polymeric species (Al-6-Al-10 species) at pH 5.0. In the same way, small and median polymeric aluminum species further aggregated into large polymeric species (Al-11-Al-21 species). When pH was up to 5.8, metastable median and large polymers species decomposed into small aluminum species, then further disaggregated into dimeric species. With pH increased to 6.4, majority of aluminum species formed to Al(OH)(3) amorphous flocs. Accordingly, coagulant hydrolysis mechanism from polymerization toward decomposition was proposed. Furthermore, formation and decomposition of polymeric aluminum species in AlCl3 solution followed the "Core-links" model, while those of Keggin-Al-13 species in polyaluminum solution was based on the "Cage-like" model. (C) 2008 Elsevier Inc. All rights reserved.