We report the cathodic electrodeposition of cerium oxide thin films from cerium-chloride aqueous solutions on p-type Si(001) substrates under illumination. Transmission electron microscopy coupled with electron diffraction analyses show that the deposits exhibited a nanocrystalline structure with a significant amorphous fraction when a hydrogen peroxide additive is used. X-ray photoelectron spectroscopy analyses were used to evaluate the surface stoichiometry, revealing a significant reduction of Ce(III) replacing Ce(IV) ionic sites for deposits prepared from solutions containing hydrogen peroxide as an additive. Adherent, uniform, and transparent CeO2 films with controllable stroichiometry can be deposited on Si, which is what renders these deposits versatile candidates for technological applications. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3240593] All rights reserved.