Pore-filled cation-exchange membranes containing poly(styrenesulfonic acid) gels have been prepared by a two-step procedure involving thermally initiated radical copolymerization of styrene and divinylbenzene in the pores of a microporous poly(ethylene) host followed by treatment with chlorosulfonic acid. The incorporated poly(styrenesulfonic acid) was shown to be evenly distributed throughout the thickness of the host membrane. A pore-contraction of the host membrane was observed during the incorporation of the cross-linked poly(styrene); however, the sulfonation step led to increases in the thicknesses and/or other dimensions of the membranes depending on mass loading of the incorporated polyelectrolyte. The ion-exchange capacities of the resulting membranes ranged up to 5.2 meq/g, depending on the mass of poly(styrene) initially incorporated into the host as well as the sulfonation temperature. The membranes were characterized in terms of water content, fixed charge concentration, and transport numbers. The pressure-driven performance of the membranes in reverse osmosis/nanofiltration applications was examined.