Electrochemical passive layers on binary Fe/Si allo ys with different Si contents of the bulk [2-50 atomic percent (a/o)] were prepared in an alkaline (pH 9.0) and in an acidic (pH 5.0) electrolyte and investigated with X-ray photoelectron and ion scattering spectroscopy. The preparation of the sputter-cleaned samples and their transfer to the Ultrahigh vacuum was performed with exclusion of the laboratory atmosphere in purified argon. The anodic films reveal a structure which reflects the dissolution properties of the pure oxides. In acidic solutions (pH 5.0) Si(IV) oxide is the predominant component, whereas at pH 9,0 large Fe contributions are found, Fe(II) at low and Fe(III) at high potentials. A Si content of greater than or equal to 21.3 a/o reduces the active dissolution and extends the passive potential range toward negative values even in acidic electrolytes in close agreement with a dense Si(IV) oxide film.