Electrochromic iridium oxide thin films were prepared by using a simple and inexpensive spray pyrolysis technique onto fluorine doped tin oxide (FTO)-coated glass substrates, from iridium chloride solution. The substrate temperature was varied between 250 and 400 degrees C. The as-deposited samples were amorphous. The electrochromic properties of thin films were studied in aqueous electrolyte (0.5N H2SO4) using cyclic voltammetry (CV), chronoamperometry (CA) and spectroelectrochemical techniques. The films exhibit anodic electrochromism upon intercalation and deintercalation of H+ ions. The colouration efficiency at 630 nm was calculated and found maximum for 1250 sample, owing its hydration. (c) 2004 Elsevier B.V. All rights reserved.