X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been used to characterize physical structure of IrO2+Ta2O5; films over the whole composition range by thermodecomposition of chloride solutions heated at 450degreesC. Solid solubilization between Ta component and IrO2 rutile in the mixed films was measured, and three typical surface morphologies of the oxide coatings were observed. The surface electrochemical properties of Ti/IrO2-Ta2O5; electrodes were studied by cyclic voltammetry at varying potential scan rate, and a 'double-layer' electrochemical structure containing the 'inner' and 'outer' layers has been distinguished. The voltammetric charge appears to decline with the decrease of grain size of oxide coatings as a result of the effect of surface tension. However, the coatings of 70% IrO2+30% Ta2O5 with the finest grains still exhibit the highest apparent activity for oxygen evolution evaluated by the anodic current at a constant potential. This result is interpreted by the measurements of open-circuit potential (E-oc) and double-layer capacitance (C-dl) using electrochemical impedance spectroscopy (EIS). Thereby, the reliability of voltammetric charge obtained in 'double-layer' potential region in determining the real electrocatalytic activity for O-2 evolution has been discussed. (C) 2003 Kluwer Academic Publishers.