A composite membrane was prepared by the combination of polypyrrole with a porous-polypropylene film, and its permselectivity was investigated. The permeability and selectivity of various sodium salts through the membrane were changed with an applied potential. The permeation of anions at the positive potentials was due to the migration through the positive sites of the polypyrrole layer, as well as the diffusion from the difference of concentration and potential. The contribution of the migration effect to the anion permeation was very large. The main factors dominating the permeation character of different anion species were the size and valence of the anions. The composite membrane worked effectively as a functional separation membrane to control the ionic permeability by potential application.