Polyaniline (PANI) membranes symmetrically bathed by electrolyte solutions on both faces have been studied by ac impedance. The membrane impedance is very sensitive to both solution pH and membrane potential, being lower under the conditions for which PANI is an electronic conductor: it increases by several orders of magnitude when pH is increased from 0 to 7 and less markedly upon membrane reduction. In the doped state, the impedance diagrams of PANI membranes have the shape of a diffusion impedance. The experimental data are analyzed with reference to a published transmission-line model. The ionic diffusion coefficient values derived from this analysis are compared with those obtained by permeation experiments.