In order to prepare a free-standing polypyrrole film with high mechanical strength, Fe(III) form or Cu(II) form cation exchange membranes were immersed in an aqueous pyrrole solution. Polymerization speed of pyrrole in the Fe(III) form membrane was faster than that in the Cu(II) form membrane. A linear relationship was observed between conductivity of the composite membrane and immersion period in the pyrrole solution. Conductivity of the composite membrane from the Fe(III) form membrane reached about 10(-3)Scm(-1), which was the ultimate value of this method. Properties of formed polypyrrole in the membrane matrix was discussed in relation to weight increase of the membrane and species of oxidative cation. Burst strength of the composite membrane increased with increasing polymerization period of pyrrole.