Anodic iridium oxide films (AIROF’s) can be grown and operated on transparent substrates. Using SnO2‐coated glass as the substrate we can monitor large, rapid, and persistent variations of the light intensity transmitted through the AIROF. The voltammogram of the AIROF on SnO2‐coated glass is essentially identical to that of an AIROF on iridium. This proves that the electrochemistry producing the coloration does not involve the substrate.