Iridium oxide thin films were deposited onto the glass substrates by spray pyrolysis technique (SPT) using aqueous solution of IrCl3.3H(2)O at various substrate (deposition) temperatures ranging from 250 to 400 degreesC. Emission of both physisorbed and chemisorbed water takes place during thermal decomposition process and Ir2O3 forms below 600 degreesC, above which dehydrated IrO2 formation takes place. The as-deposited samples at all substrate temperatures were X-ray amorphous and those annealed at 600 degreesC for 3 h in milieu of air were polycrystalline tetragonal IrO2. The infrared (IR) and Raman spectroscopic results echo above findings. The optical and electrical characteristics reveal that optical transition takes place through d-d intraband excitation and p-type semiconductor to metallic transition at 600 degreesC. The semiconducting to metal transitions in iridium oxide thin films were discussed on the basis of Anderson's theory. (C) 2002 Elsevier Science B.V. All rights reserved.