Polystyrene cation exchange membranes were prepared by a PVC-based semi-interpenetrating polymer network (IPN) method. The reaction behaviors during polymerization and sulfonation in the preparation method were investigated. The prepared membranes were characterized in terms of the physical and electrochemical properties. The membranes exhibited reasonable mechanical properties (tensile strength, 13 MPa, and elongation at break, 52%) for an ion-exchange membrane with the ratio of polystyrene-divinylbenzene (DVB)/poly(vinyl chloride) (PVC) (RSt-DVS/PVC) of below 0.9. Fourier transform infrared/attenuated total reflectance, differential scanning calorimetry, and scanning electron microscopy studies revealed the formation of a homogeneous membrane. The resulting membrane showed membrane electrical resistance of 2.0 Omega cm(2) and ion-exchange capacity of 3.0 meq/g dry membrane. The current-voltage (I-V) curves of the membrane show that the semi-IPN polystyrene membranes can be properly used at a high current density, and that the distribution of cation-exchange sites in the membrane was more homogenous than that in commercial membranes. (C) 2003 Wiley Periodicals, Inc.