The barrier properties of a poly(aniline) membrane may be tuned by electrochemical control of the polymer's oxidation state. We have exploited this behavior to electrochemically modulate the permeation of two probe molecules of differing charge through a supported poly(aniline) membrane. The trans-membrane flux was measured for neutrally charged phenol and negatively charged 4-hydroxybezenesulfonate as a function of the polymer oxidation state, which was controlled by stepping the potential applied to a poly(aniline) film from a reducing potential to increasingly more oxidizing values. The fluxes of phenol and of 4-hydroxybezenesulfonate were enhanced when the polymer was poised in the half-oxidized, emeraldine state as compared to that of the reduced, leucoemeraldine state. Additional film oxidation to the pernigraniline state produced an enhanced flux of 4-hydroxybenzenesulfonate over phenol. The flux responses for these probe molecules are explained through a combination of oxidation-induced film swelling and electrostatic effects due to variable film charge. At high membrane potentials, film modulation was disrupted by poly(aniline) hydrolysis and decomposition. This work demonstrates that the permeation of a supported poly(aniline) membrane may be modified electrochemically to produce controllable barrier properties.