Polyacrylonitrile (PAN) membranes display some unusual features for ultratiltration (UF). The meso-macropores of PAN UF membranes can be easily reduced into the range of micro-mesopores by taking advantage of surface tension forces within the capillary pores during heat treatment in the presence of ZnCl2. Asymmetric PAN nanofiltration (NF) membranes with controlled highly dense pore surface functional groups were prepared by hydrolysis of the nitrile groups with NaOH. The combined effects of heat treatment and the presence of ZnCl2 on the formation of nanofiltration membranes were investigated. In addition, membrane post-treatment with NaOH was studied. The effect of counterion species on the membrane performance was also investigated. A simple new method, which utilized univalent alkaline ions as probes, was developed to derive the average pore size of cationic nanofiltration PAN membranes from pure water permeability coefficients. Published by Elsevier B.V.