The electronic conduction mechanism through the electrochemically formed passive film on an iron polycrystalline surface was studied by in situ electrochemical scanning tunneling microscopy (EC-STM) and electrochemical impedance spectroscopy (EIS). In situ STM imaging conditions of the iron oxide layer in the different electrochemical potential regions [Fe(II) and Fe(III) oxide] were determined and successfully compared to their semiconducting properties evaluated by EIS. As a result, a quantitative explanation for the highly stable STM images of a thick iron oxide formed at high anodic potentials can be obtained. This experimental data allows us to present a quantitative energy diagram of the iron\passive film\electrolyte interface as well as to give first estimates of some electronic parameters of the passive film on iron. These results suggests that the so-called passivity of iron electrodes may arise from the buildup of an energy barrier at the oxide\electrolyte interface. (C) 2003 The Electrochemical Society.