Polyaluminum chloride (PAC) is an important flocculant in potable water and wastewater treatment. Al-13 or Al-b is regarded as the most efficient species of PAC. A low base addition rate, small base drop sizes, and sufficient mixing usually favour the formation of PAC with higher Al-b contents. In this paper, PAC was originally synthesized with a membrane reactor in which NaOH solution permeated through the micropores of an ultrafiltration membrane into AlCl3 solution gradually to reduce the NaOH droplets size to nanoscale, about 10(6) times smaller than that of conventional methods, resulting in a great reduction in the local supersaturation and precipitates generated and, subsequently, in an increase in the Al-b content. The effects of the membrane molecular weight cutoff (MWCO) and reactant concentration on the species distribution were investigated. It was found that Al-b increases with decreasing MWCO and reactant concentration under the experimental conditions used. A new plausible reaction pathway is also proposed that assumes that the hydrolysis and polymerization of Al3+ constitute a complex net of consecutive and parallel reactions and that the species distribution is mainly determined by the reactions kinetics.