Ellipsometry is used as a. monitoring technique to study the galvanostatic growth and the open circuit dissolution Of WO3 films in different acid media. The optical properties of the oxide in the presence or the absence of the electric field across the film were obtained. It is shown that ellipsometry is a suitable technique for distinguishing among different phenomena like hydration and generation of porosity and/or roughness occurring simultaneously with the decrease of thickness during dissolution. WO3 dissolution obeys a zero-order rate law with respect to the film thickness. (C) 2003 Elsevier B.V. All rights reserved.