Bilayer lipid membranes tethered to planar gold electrodes were prepared, based on a self assembled monolayer (SAM) of 2,3-di-O-phytanyl-sn-glycerol-l-tetraethylene glycol-DL-alpha-lipoic acid ester lipid (DPTL). When the SAM's were exposed to a suspension of liposomes made from diphytanoylphosphodatidyl choline (DPhyPC), tethered lipid bilayers (tBLMs) were formed with good sealing properties. The preformed tBLMs were doped with valinomycin, and the K+ ion concentration in the bathing solution was increased stepwise by adding KCl. Electrical impedance spectra were recorded following every addition. These data were modelled by means of the network simulation program SPICE, using parameter values of the undoped membrane and a kinetic scheme for the K+/valinomycin system, whose rate constants were determined previously in independent measurements. Experimental and simulated data are in reasonable agreement despite some simplifying assumptions used in the SPICE simulations. Experimental data were also fitted to a conventional equivalent circuit, which reveals that the representation of the K+/valinomycin system by an ohmic resistor can be considered as no more than an approximation. (C) 2003 Elsevier Science B.V. All rights reserved.