The bulk and interfacial transport properties of PANI free-standing membranes have been investigated in acidic media with chloride as common counter-ion and with different co-ions, using the a.c. impedance technique with a four-electrode cell. The experimental data were compared to a recently proposed theoretical expression, based on use of the Nernst-Planck equation for mass transport and on slow discharge for ionic interfacial transfer, Though the only transport properties addressed in this model are the counter-ion and the polaron mobilities, experiments show an influence of the co-ion; In the case of multiple charge cations, especially Zn2+, all mobilities are strongly reduced.