The electroreduction of thin Bi2O3 films was characterized by employing electrochemical techniques and in situ ellipsometry. The electroreduction optical response is described in terms of a bilayer model. The optical analysis indicates that electroreduction proceeds yielding a rough/porous layer of metallic bismuth at the oxide/electrolyte interface, that grows inwards at the expense of the underneath oxide layer to completely exhaust it. At the end of oxide electroreduction, the aging process of the porous metallic bismuth layer could be detected. On this framework, changes in the oxide growth kinetics observed on electroreduced surfaces can be consistently explained. (c) 2005 Elsevier B.V. All rights reserved.