The properties of cation-exchange membranes based on polymeric composites of sulfonated poly(styrene-co-divinylbenzene) and polypyrrole (Ppy) are discussed. The amount of Ppy in the composites depends on the oxidant solution concentration used in the synthesis. Comparing composite membranes, the highest electronic conductivity and the best thermal stability were obtained for the composite membrane with the greatest content of Ppy. The membranes were thermally stable up to 250 degreesC. As could be seen by SEM, Ppy is homogeneously distributed in the sulfonated poly(styrene-co-divinylbenzene) matrix. Consequently, composite membranes swell in water much less than base membrane alone. Ion-exchange capacity (IEC) does not change with pyrrole polymerization.