Polyaluminum chloride (PAC) with a high content of Al-b, the most efficient species of PAC, was synthesized with a membrane reactor. In this reactor, NaOH solution permeated slowly through the micropores of an ultrafiltration membrane into AlCl3 solution to reduce the NaOH drop size to nano-scale. This resulted in a dramatic reduction of localized supersaturation and enhancement of the Al-b quantity. In this paper, a constant flow pump was employed to realize steady permeation fluxes of NaOH. The effects of permeation fluxes of NaOH, flow rates of AlCl3 solution, permeation flux alteration operation and bi-directional flow operation were studied in detail. The results show that lowering the permeation flux of NaOH and enhancing the flow rate of the AlCl3 solution were favorable for the improvement of the Al-b content in the product. Slowing down the base-addition rate when basicity (molar ratio of OH/Al) was over 1.5 increased the maximum Al-b content; speeding the permeate flux when B < 1.5 shortened the synthesis period. Besides, the bi-directional flow operation was proven to be favorable for the increment in the Al-b content. (C) 2003 Elsevier B.V. All rights reserved.