The surface characterization of IrO2-Ta2O5 coated titanium electrodes has been studied in a 0.5 mol dm(-3) Na2SO4 solution using cyclic voltammetry. The results showed that the surface state of all the electrodes is controlled by the redox transition of Ir(III)/ Ir(IV). The voltammetric charge, the double-layer capacitance, and the electrochemical activity present similar variation with the composition of the oxide mixture, showing maximum values at 50-70% Ir. The high electrochemical activity of the electrode with 50-70% Ir is attributed to not only its good electrocatalytic effect but also its large electrochemically active surface area, especially the contribution of the inner surface. (C) 2003 The Electrochemical Society.