Amorphous Ni-Cr-P-B alloys, which possess a high corrosion resistance and are apt to form the amorphous structure, have been prepared by rapid quenching from melts. The alloys containing chromium of 7 and 9at.% are not corroded in 10% FeCl₃·6H₂O at 30°C. Spontaneous passivation occurs on these alloys in 2N H₂SO₄ and 1N HCl, and a critical potential for pitting is not observed on the anodic polarization curves even in 1N HCl. Simultaneous determination of the thickness and composition of a passive film and the composition of the surface of substrate alloy directly under the passive film has been carried out by X-ray photoelectron spectroscopy. The passive film formed in 2N H₂SO₄ consists mainly of hydrated chromium oxy-hydroxide similar to those formed on corrosion-resistant crystalline alloys containing chromium and the composition of the surface of substrate alloy directly under the film is not different from that of bulk alloy. On the contrary, the composition of passive film formed in 1N HCl can be expressed as (Ni^II_0.26Cr^III_0.74) (PO₄)_0.74O_x (OH)_0.52-2x (1.74+x) H₂O and hence phosphate comprises 81% of constituents of the passive film. In addition, the composition of the surface of substrate alloy directly under the film changes from that of bulk alloy. In conclusion, the amorphous Ni-Cr-P-B alloys with chromium of 7 to 9at.% have a high corrosion resistance in strongly acidic solutions and the great resistance to pitting corrosion, but may not be always stable for a long time because of the change in alloy composition directly under the film.