Selectivity between monovalent cations and its sequence of conductivity in lipid bilayers doped with the antibiotic Gramicidin D (GD) were examined using EIS. Experiments were performed using lipid bilayers obtained from a lipid mixture of phosphatidylcholine and dimethyldioctadecylammonium chloride (DODAC). Lipid bilayers were supported on gold surfaces modified with a mercapto-carboxylic acid. The bilayers were formed by chemisorption of this last species to form the first monolayer on gold and subsequent fusion of unilamellar vesicles to form an external bilayer attached by electrostatic interactions. A mathematical expression for the impedance of the membrane processes was derived. Some predictions of the presented model were checked after fitting the experimental results in various electrolyte compositions.