We examine the electrochromic effect of anodic iridium oxide films (AIROF's) in aqueous and nonaqueous electrolytes. All experimental data in aqueous electrolytes are consistent with the hypothesis that electrochromism in AIROF's can occur via hydroxide ions, without insertion‐extraction of protons from the electrolyte. We also report new experiments in which electrochromism of AIROF's takes place by insertion‐extraction of anions (OH−, F−, CN−) from nonaqueous electrolytes.