The corrosion resistance of amorphous Fe-8Cr-P alloys depends very strongly on the amount of phosphorus addition. The corrosion rates of crystalline Fe-8Cr-7P and Fe-Cr-13P alloys in 9 M H2SO4 solution at 30°C are extremely high in comparison with those of amorphous Fe-8Cr-P alloys with 16.5 at% or more phosphorus. An increase in the phosphorus content improves the corrosion resistance. The amorphous Fe-8Cr-16.5P and Fe-8Cr-18P alloys which are composed of very fine bcc phase dispersed in the amorphous matrix turn black after immersion for about 10 min, while the amorphous Fe-8Cr-20P alloy maintains the metallic luster after immersion for 168 h. Intense spectra of elements in the metallic state are observed in XPS analysis even though the specimen surface turns black. This metallic species found by XPS corresponds to the amorphous Fe-Cr-P phase which is not corroded. The thick corroded layer formed on Fe-8Cr-16.5P and Fe-8Cr-18P alloys seems to be the amorphous alloy phase after dissolution of the microcrystalline bcc Fe, the amorphous Fe-Cr-P phase being covered with a passive film. The amorphous Fe-8Cr-20P alloy does not show initial active dissolution, and the corrosion loss of this alloy is very small. Therefore, at least 20 at% addition of phosphorus to the Fe-8Cr alloy is necessary to form the completely amorphous structure and to provide the high corrosion resistance.