The ultra-low pressure water softening properties of a series of novel membranes containing cross-linked poly(4-vinylpyridinium) salts incorporated into the pores of polyethylene or polypropylene microfiltration membranes have been examined. It has been shown that these membranes exhibit excellent separation properties with high permeabilities at low trans-membrane pressures. The separation mechanism in these pore-filled membranes is primarily due to Donnan exclusion, and as such bivalent cations are more strongly rejected than monovalent ions. Moreover, it has been shown that relatively large molecules such as sucrose are only rejected to a limited extent. The membranes have been shown to withstand free chlorine at ambient temperatures. The performance of the pore-filled membranes has been shown to compare favorably with existing state of the art thin-film composite (TFC) nanofiltration membranes (Osmonics Desal-51 and BQO1 and Hydranautics TFV-7450). In contrast to the TFC membranes, the separating layer in these pore-filled membranes has been shown to be thick, ranging from 80 to 160 mu m. These polyelectrolyte pore-filled membranes offer a new paradigm in the construction of high performance water softening membranes.