Iron oxide/oxyhydroxide thin films are currently being investigated for their applications in photoelectrochemical solar cells for solar energy conversion, sensors, catalysts, Li batteries, and electrochemical capacitors. The fabrication of nanoarchitectural thin films is very important for those applications because of the size-related effects of their performance. This study reports the novel synthesis of nanosized cellular iron oxide/oxyhydroxide thin films with high degrees of crystallinity and purity by electrochemical deposition of green rust thin films and their chemical oxidation. This technique is applicable to substrates with large surface areas of a few square centimeters or above without the aid of templates, high temperature, vacuum, or post-treatment. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1945647] All rights reserved.