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In situ observation of initial process of rust formation on Fe-5 mass%Cr and Fe-9 mass%Ni alloys using SPring-8 synchrotron radiation was carried out under wet/dry cycling with 3.5% Na 2 SO 4 and NaCl solution films. It was shown that the rust constituents detected during the quite initial wet/dry cycles were Fe(OH) 2 and Fe(OH) 3 independent of alloying elements. Most characteristic X-ray diffraction peaks obtained under the Na 2 SO 4 solution film are indexed as those from goethite, -FeOOH, both in the Fe-Cr and Fe-Ni alloys. In the case of the Fe-Cr alloy, coexistence of Cr 3þ ion in the Na 2 SO 4 film weakened the -FeOOH diffraction peaks probably due to formation of ultrafine goethite crystals. Akaganéite, -FeOOH, appears only under the NaCl solution film. Especially, the Fe-Ni alloy forms a considerable amount of akaganéite, which can be interpreted in terms of formation of the monoclinic akaganéite.