The thickness and chemical composition of passive films formed on Fe-Ni alloys with various Ni contents were estimated by means of coulometry at potentiostatic-anodic oxidation and galvanostatic-cathodic reduction. The amounts of Fe²⁺ and Ni²⁺ dissolved during the oxidation and reduction were determined by colorimetric analysis. The passive films were found to consist of a mixture of Fe₃O₄, γ-Fe₂O₃, NiO and Ni₃O₄. At the potentials over 800 mV, NiO₂ was formed from Ni₃O₄. The thickness of the passive films increased almost linearly with the anodic potential and was estimated to be 15∼81 Å. The film thickness was also found to increase with increasing Ni content, showing a critical value at 30 Ni wt%. This tendency is similar to the dependence of corrosion resistivity on Ni content. The Ni content in surface oxide films enriched during the passivation in lower pH solutions, while no enrichment occurred in higher pH solutions.