Commercial ion-exchange membranes are usually thick, and have high electrical resistances that can reduce the adsorption capacity of electrodes in membrane capacitive deionization (MCDI) cells. In this study, porous polyethylene (PE) membranes, filled with a sulfonated monomer, crosslinking agent (5, 10, 15 wt%), and radical initiator, were subjected to polymerization and subsequent ion-exchange, to yield PE filled with crosslinked sulfonated polystyrene (PE-CSPS-5, 10, 15). PE-CSPS membranes were very thin (27 [un), and exhibited moderate water uptake (26-35 wt%) and ion-exchange capacity (0.70-1.0 meq/g), and low electrical resistances (0.33-0.62 Omega.cm(2)) compared with commercial ones. MCDI cells were assembled with either a PE-CSPS-5 membrane or a commercial cation-exchange membrane (CMX, IEC 1.5-1.8 meq/g), and their performances studied using a 500 ppm solution of NaCl. The current efficiency of the PE-CSPS-5-based MCDI cell showed slightly higher than that of the commercial CMX-based MCDI cell, which was attributed to the considerably lower electrical resistance of the membrane (0.33 vs. 2.9 Omega.cm(2)), even though its IEC value is smaller (1.0 vs. 1.5-1.8 meq/g). Our study reveals that supporting membranes filled with ion-exchange polymers have advantages over casting membranes in preparing membranes for MCDI application, with respect to dimentional stability and thickness.