Direct measurements of the H+ transport through polyaniline (PAN) membrane separating two aqueous solutions were conducted. It was demonstrated that Pick's law could not describe the Ht transport through PAN membrane. It was found that the transport of H+ from acidic to neutral solution through PAN membrane (similar to 30 mu m thickness) reaches its maximum steady state value after relatively long time. Time lag was 400 min at pH = 2 in the donor solution. This time lag value showed a strong dependence on the donor Ht concentration and decreased at lower pH. The steady state transport rate was less than 1.0 x 10(-9) mol/s cm(2) if the H+ concentration in the donor solution was less than 0.5 M, but then it increased sharply at higher [H+] to the level above 1.0 x 10(-8) mol/s cm(2). A kinetic model, taking into account coupled H+/anion transport, ion interactions with PAN polymer chains and their structural changes, is proposed. The large value of the time lag is explained on the basis of the idea about slow structural changes in the film during the H+ induced doping.