The oxidation state of iridium ions in an oxide film grown electrochemically on an Ir metal reflector electrode in aqueous 0.5MH2SO4electrolyte can be rapidly and reversibly modulated by voltage pulses of 1 V amplitude according to the reaction: Ir(OH)n(transparent) &rlarr2;IrOx(OH)n−x(colored)+xH++xe. This valency interconversion produces a marked change in light absorption throughout the visible wavelength region and occurs without change in film thickness. Color‐bleach cycles exhibit reflectance contrast changes, &Dgr;R/R, and charging times, &tgr;, suitable for electrochromic display devices, e.g., for a film 700 A˚ thick: &Dgr;R/R=60% at &lgr;=546 nm and &tgr;≈40 msec. The fast write‐erase times are made possible by the highly porous and hydrated nature of the oxide film. The charge (∼20 mC cm−2) and energy (∼20 mJ cm−2) are comparable with those for other electrochromic oxide systems, e.g., the tungsten bronzes. Advantages of the iridium oxide system include: (i) fast response; (ii) stability of the colored written state in the presence of water and dissolved O2; (iii) broad spectral absorption; and (iv) the ability to grow and reform the oxide layerinsituin the electro‐optic display cell. Electrochromic spectra of anodic iridium oxide films are illustrated and related to charge‐transfer and intervalency transitions.