A tethered bilayer lipid membrane (tBLM) was prepared by anchoring a thiolipid monolayer to a Ag(l 11) single crystal from a thiolipid solution in ethanol; a diphytanoylphosphatidy1choline monolayer was then self-assembled on top of the thiolipid monolayer from a lipid solution in hexane or ethanol. The thickness of the mixed thiolipidilipid bilayer was estimated at 7.5 0.5 nrn by scratching holes in the tBLM with an AFM tip and determining their height profile and the frequency distribution of height. The effect of the incorporation of gramicidin A and valinomycin upon K' ion transport across the tBLM was investigated by electrochemical impedance spectroscopy. The position of the inflection point of the sigmoidal curve of the in-phase component of the electrode admittance against potential allowed the determination of an approximate scale of the absolute potential difference across the whole Ag(1 1 1)vertical bar(aqueous solution) interface, on the basis of a model of the electrified interface. (c) 2008 Elsevier Ltd. All rights reserved.