Freshly prepared RuO2-Ta2O5 thin films containing between 10 and 80 atom% Ru have been examined and characterized by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and morphological analysis (SEM - scanning electron microscopy/EDS - energy dispersive X-ray spectroscopy). Investigation of the electrical properties, charging process, and passivation of the electrode containing RuO2-Ta2O5 thin films was conducted as a function of electrode composition in a 0.5 mol dm(-3) H2SO4 solution. For potential values in the double layer region (0.2-1.0 V vs. RHE), the impedance profile observed at low frequency domain was attributed mainly to the capacitive behavior of the oxide/solution interface. As for the high frequency domain, the impedance profile gave evidence that the kinetic process is limited by supporting electrolyte/water diffusion inside the pores of the difficult-to-access oxide regions and/or the Ti/oxide interface. The electrode passivation mechanism toward OER (oxygen evolution reaction - 1.5 V vs. RHE) was also investigated during long-term electrolysis (j = 750 mA cm(-2) and T= 80 degrees C) by means of EIS at pre-established times. The SEM-EDS data give evidence of the increase in the TiOx interlayer. Moreover, the EIS data furnished complementary insight that helped our proposition of the deactivation mechanism. (c) 2006 Elsevier B.V. All rights reserved.