Silver sulphide/lead sulphide membranes were studied using electrochemical impedance spectroscopy. The influence of the electrolyte concentration and the membrane thickness were evaluated. The complex impedance plots have shown two capacitive loops: one at high frequency range, related with the charge transfer resistance at the interface membrane-Ag and a second one at low frequency range, associated with the diffusion process through the membrane. A simple model was used to take into account the experimental results: the changes of the potential with the time and the electrolyte concentration; the changes of the charge transfer resistance and the diffusion resistance with the electrolyte concentration. An empirical equation was used to calculate the diffusion coefficient of Ag+ inside the membrane. (c) 2005 Elsevier Ltd. All rights reserved.