The thermolytic formation of IrO2 + Ta2O5 mixed oxides from chloride precursors is studied by thermogravimetry (TGA) and differential thermal analysis (DTA). The structure and morphologies of the corresponding oxide films coated on titanium bases are determined by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM), respectively. The experimental results showed that, as a result of the interaction between Ir and Ta components, especially, the formation of solid solution phases during the thermolysis processes, the oxidative dissociation of the H2IrCl6 + TaCl5 mixture is facilitated. The catalytic effect reached the maximum at a nominal IrO2 content of 70 mol% in the expected product, i.e. IrO2 + Ta2O5 mixed oxides, accompanied by the highest solid solubility between the two oxides and the finest rutile-structured crystalline grains in the oxides. For the mixed precursors with a low iridium content (e.g. 10 mol% nominal IrO2 in IrO2 + Ta2O5) or a low tantalum content (e.g. 80 mol% nominal IrO2), however, the decomposition of the major component is inhibited by the minor one at high temperatures (610-800 degreesC). The results show that the solid solution at low Ir contents (<30 mol% IrO2) is unstable since it decomposes at high temperatures (greater than or equal to750 degreesC). Two or more IrO2 based rutile-constructed solid solution phases are thermolytically formed from the mixed precursors with nominal IrO2) contents greater than or equal to 30 mol%. The rutile-structured phases stably exist only in the case of IrO2 contents greater than or equal to60 mol%. (C) 2003 Elsevier Science B.V. All rights reserved.