The electrochemical behavior of synthetic Cr- and FE-oxide films with well known chemical composition and structure, prepared by sputter deposition, has been studied. Photoelectrochemistry and Mott-Schottky analysis show that the parameters during the sputter deposition process influence the stoichiometry of Cr- and Fe-oxides. The bandgap energy of Cr-oxides is independent of the solution pH, thus the values of the bandgap energy lower than those found for ideal Cr2O3 are due mainly to nonstoichiometry. The defect structure influences the electrochemical reactivity of the Cr-oxide. The photocurrent behavior of Fe3+-oxides is strongly determined by the presence of Fe2+ in the oxide, which can be introduced by a partial electrochemical reduction of the Fe-oxides. Chloride ions did not show an effect on the electrochemical reactivity of stoichiometric Fe2O3. Mott-Schottky analysis shows changes in the capacitance behavior if chloride is added to the solution; this suggests that a specific adsorption of chloride takes place on the surface. A comparison of the stability of the synthetic oxides and natural passive films may lead to new insights into the role of stoichiometry and defects on the breakdown of passive films.