Abstract: Communication in living organisms is governed by cell bilayer membranes, which selectively recognize a specific component in the presence of others and accordingly respond. The functioning of such molecular-size barriers involves molecular and quantum processes deriving from a precise, purpose-oriented architecture, and attempts have been made to create artificial supramolecular structures exhibiting similar properties1–9. In particular, chemically modified electrodes, coated with various types of organic layers10–18, have been used to control the access of electroactive species from solution, but such systems have so far lacked some of the important features of real, molecular-size membranes. Here we present the first example of an electrode coated with a stable, ion-selective artificial membrane having the thickness of just one molecule, which successfully mimics basic structural and functional principles of the natural bilayer membrane. This monolayer membrane, produced by molecular self-assembly on gold, can recognize a selected metal ion in the presence of other ions, and thus induces a specific electrode response. It consists of synthetic 'receptor sites', designed to impart the desired selectivity, embedded within an inert monolayer matrix which blocks vacant sites on the surface and so prevents the passage of undesired species. The supporting gold electrode permits electrochemical analysis of the membrane structure and performance. Such monolayer membranes may aid the study of elementary charge transfer processes at liquid–solid interfaces, and contribute to future molecular-based technologies.