Films of iridium-tantalum oxide and iridium oxide have been prepared by sputtering and studied regarding their structure and electrochemical properties. X-ray diffraction and transmission electron microscopy showed an average grain size of 3-4 nm for both films. Point energy dispersive X-ray spectrometry showed an inhomogeneous distribution of iridium and tantalum indicating that the iridium-tantalum oxide may be a mixture of small IrO2 and Ta2O5 grains, which is consistent with the determined composition IrTa1.4O5.6. X-ray photoelectron spectroscopy gave valuable information on the stabilization process of the as-deposited films involving an uptake of oxygen, and on a coloration mechanism only including protons. (c) 2007 Elsevier B.V. All rights reserved.