Electrochromic iridium oxide (IrOx) and iridium-tantalum oxide (IrTaOx) thin films were prepared by sputtering. Complex refractive indices were determined for samples deposited on indium-tin oxide covered glass in different colouration states, and for as-deposited samples on sapphire and Corning glass. The refractive index was found to be practically constant for both IrOx (similar to 1.3) and IrTaOx (similar to 2). The extinction coefficient was found to vary between the coloured and bleached states with similar to 35% for IrOx and similar to 55% for IrTaOx at 660 nm. This is believed to be a result of the removal of intraband transitions within the Ir t(2g) band during bleaching. (C) 2008 Elsevier B.V. All rights reserved.