The rusts formed on mild steel (15-year exposure) and weathering steel (32-year exposure) exposed to an industrial environment have been characterized by means of X-ray diffraction technique and ⁵⁷Fe Mössbauer spectroscopy. By using an X-ray diffraction method, it is suggested that the rusts formed on both steels consist of the crystalline α-FeOOH, γ-FeOOH and an X-ray amorphous phase, which gives no peak to X-ray diffraction pattern. The amount of the X-ray amorphous phase exceeds 50% of the total amount of the rust. The ⁵⁷Fe Mössbauer spectra observed at 10 K indicate that the rust contains only α-FeOOH, γ-FeOOH and Fe_3−δO₄ (γ-Fe₂O₃) for mild steel, and only α-FeOOH and γ-FeOOH for weathering steel. The X-ray amorphous substance in the rust layer formed on mild steel possesses the structures of mainly α-FeOOH showing superparamagnetism owing to its small particle size, and Fe_3−δO₄ (γ-Fe₂O₃). They are contained both in the inner rust layer and in the outer rust layer. The X-ray amorphous phase in the rust layer formed on weathering steel is mainly α-FeOOH.