Polypyrrole films were prepared electro-oxidatively on Pt plate anodes in subcritical and supercritical 1,1-difluoroethane (R152a). This material can be removed from the anode as freestanding membranes. The membranes thus synthesized in subcritical and supercritical R152a exhibited clear differences in their properties; that is, the membranes have denser structures with a smooth surface and higher mechanical strengths and electrical conductivities, compared to those of the membranes synthesized in the conventional organic media such as an acetonitrile solution. Electrochemically controlled transport of metal ions such as Li(+), Na(+), and K(+) across the membranes was also investigated.