The electrochromic mechanism of IrO2 thin films was investigated with electrochemical quartz crystal microbalance and surface analyses such as X-ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy. Prior to pulsed anodic current electrodeposition of IrO2 thin film on indium-tin oxide substrate (total anodic charge 100 mC/cm(2)), IrO2 islands, which work as good adhesive seeds, were prepared by cyclic voltammetry. In phosphate buffered saline solution, IrO2 exhibited blue and black anodic electrochromic behavior at two oxidation potentials of +0.5 and +0.9 V (vs. a saturated calomel electrode). Surface analysis and mass variations of IrO2 thin film indicated that the ejection of H+ and the injection of Na+ caused the cracks in the film and resulted in coloring behaviors. (C) 2003 The Electrochemical Society.