Using an instantaneous base-feeding method and a batch-feeding method, high-concentration polyaluminum chloride (PACl) products were prepared employing CaO as the basification reagent. The distribution and transformation of the Al species of PACl was explored for the first time over a broad total Al concentration (Al-T) range. There is always an optimal basicity (B value) at which the product has the highest Al-b content, whether Al-T is high or low. The optimal B value and the highest Al-b content obtained both decrease with increasing Al-T. When the B value is lower than optimal, the overall reaction mainly leads to the production of Al-b; when the B value is higher, the reaction leads to the production of Alc. During the dilution process, high B value PACl has a tendency towards Al-b formation, while low B value PACl favors Alc formation. The transformation behaviors of Al species are influenced by Al-T in the preparation or dilution process. (C) 2009 Elsevier B. V. All rights reserved.