A novel technique allowed EIS and microscopical observation of the interface between electrode and solution to be coupled in situ. The electrochemical reduction of oxygen was investigated by using this device when the electrode surface was progressively covered by an insulating layer of scale deposit. It was shown that two time domains can be distinguished front the initial time of the experiment corresponding to a bare electrode up to a totally blocked surface. In the first part of the experiment, EIS showed one time constant which characterizes the charge transfer process of water reduction. When the percentage of coverage reached more than 80%, this latter electrochemical contribution to EIS was negligible and the reduction process of oxygen was highlighted by two time constants observed from impedance diagrams. The high frequency response characterized the charge transfer process and the low frequency loop identified the diffusion process of oxygen. An electrochemical model was proposed to explain the shape of the impedance spectra plotted with respect to the time of coverage. The good correlation between the experimental and the fitted model curves led to the change versus time of the different parameters of the equivalent circuit. It notably showed that the double layer capacity was directly proportional to the active surface measured by the microscope observation until the surface was totally covered, according to the expectations in the experimental conditions used here. (c) 2005 Elsevier Ltd. All rights reserved.