The bulk and interfacial transport properties of PANI free-standing membranes have been studied in acidic media with different counter-ions using the ac impedance technique with the fourelectrode cell. The experimental data were compared to a theoretical expression recently proposed, based on Nernst-Planck equation for mass transport and on slow discharge for ionic interfacial transfer. The ionic charge transfer resistance was found negligible for all counter-ions in acidic pH. For each system, the polaronic diffusivity was in general between one and two orders of magnitude higher than the ionic one. A classification from the fastest to the slowest ionic species was established following the order: chloride, sulphate, nitrate, phosphate and perchlorate.