Electrical conductivity experiments were carried out in a bi-ionic (chloride-nitrate) electrolyte with and without a permselective anion exchange membrane (ACS of Tokuyama Soda) at variable chloride/nitrate concentration. Diffusion coefficients of counter ions (Cl- and NO3-) in the membrane were determined from a mathematical model based on the Nernst-Planck diffusion-convection theory: the mean diffusion coefficients in the membrane were obtained by fitting the (conductivity vs. concentration) curves with the Nernst-Planck equation. (C) 2003 Elsevier Science Ltd. All rights reserved.