The impedance of the anodically formed hydrous Ru oxide in the system Ruloxide film\M HClO4 solution has been studied in the range of potentials where the electrode process occurs by a double electron and proton exchange between the oxide film and the solution. The results allowed us to clearly distinguish between the surface process at higher frequency and the bulk process at lower frequency. The high-frequency charging is found to be coupled to Faradaic charging at the film/solution interface. Evaluation of the impedance data at lower frequency, using diffusion equations for the finite boundary conditions, yields an effective proton diffusion coefficient to be 10(-10) to 10(-11) cm(2) s(-1). Oxygen reduction on the spontaneously oxidized ruthenium electrode was discussed on the basis of a rotating ring-disk voltammetry. (C) 2005 Elsevier Ltd. All rights reserved.