Effects of alloying elements (Ni and Mo) on the structure of passive film formed on Fe-20Cr alloys in pH 8.5 buffer solution were explored by analyzing the in situ electronic properties measured using the photoelectrochemical technique and Mott-Schottky analysis. The passive film formed on Fe-20Cr-10Ni was found to be mainly composed of Cr-substituted gamma-Fe2O3 from similarities in photocurrent response for the passive films formed on the alloy and Fe-20Cr. On the other hand, the photocurrent spectra for the passive films of Fe-20Cr-15Ni-(0,4)Mo alloys exhibited the spectral components associated with NiO and Mo oxide (MoO2 and/or MoO3) in addition to that induced by Cr-substituted gamma-Fe2O3. Mott-Schottky plots for the passive films formed on Fe-20Cr-(10,15)Ni and Fe-20Cr-15Ni-4Mo confirmed that the passive films on Fe-20Cr-(10,15)Ni-(0,4)Mo alloys have a base structure of Cr-substituted gamma-Fe2O3 with variation of densities of shallow and deep donors depending on the Ni and Mo contents in the alloys. We suggest that the passive film formed on Fe-20Cr-(10,15)Ni and Fe-20Cr-15Ni-4Mo alloys are composed of (Cr, Ni, Mo)-substituted gamma-Fe2O3 when the concentrations of Ni and Mo are below critical values. However, NiO and Mo oxide (MoO2 and/or MoO3) would be precipitated in the passive films when the concentrations of Ni and Mo exceed critical values. (c) 2006 Elsevier B.V. All rights reserved.