Equations describing current and concentration of electroactive species at constant potential in a thin electrolyte layer at a disc electrode were derived on the assumption that the current is controlled either by the rate of the electrode process or by the rate of diffusion. Measurements of the anodic diffusion current in a solution of 0.005 M K4Fe(CN)(6) in 1 M KCl in a cell with a controllable thickness of the electrolyte layer showed the possibilities and limitations of the method.