Hydrous iron oxide layers electrochemically formed on iron electrodes in different electrolytes (8.9 < pH < 12.6) have been investigated by ellipsometry. Data have been interpreted on the basis of a duplex oxide layer structure consisting of an inner barrier layer and an outer hydrous layer. The optical response of the hydrous oxide layer depends on the solution composition and in particular on the presence of carbonate ions and the applied potential.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas