dc-resistance measurements of metal thin films can be used to investigate processes at the solid-vacuum and solid-electrolyte boundary. In electrochemistry this is a new method to determine the quantity of adsorbed species on solid electrodes. We have grown an epitaxial Ag(111) thin film electrode on a Si(111) substrate with stable characteristics under electrochemical conditions [1]. The adsorption and desorption of small quantities of metal cations from aqueous solutions leg 1 mu M Pb2+) are detected by changes of the de-resistance, resulting from a diffuse scattering of conduction electrons by the adsorbates. In contrast to polycrystalline silver films epitaxial Ag(111) thin film electrodes allow us to perform many deposition and stripping cycles of lead-without changing the properties of the thin film.