Modulated reflection spectra of passive films on a series of Ni-Cr alloys (5 to 28 mass % Cr) have been measured in 1 kmolm-3 Na2SO4 of pH 6.0. Quantitative relations between the characteristic of spectra and the composition of the films have been derived from spectra for thin composite films of NiO-Cr2O3 artificially prepared on Pt by MO-CVD technique. From these relations, cationic mass fractions of Cr3+ ions, XCr, of passive films on the alloys were determined. It was found that passive

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... on the alloys were composed of Ni(II) and Cr(III) oxides, and that their cationic fractions of Cr3+ ions changed remarkably with Cr content of the alloy. That is, the values of XCr for the films formed at 0.10V increased from 0.10 for Ni-5 mass % Cr alloy to 0.88 for Ni-28 mass % Cr alloy with increasing Cr content of the alloy. This indicates that Cr3+ ions were substantially enriched in the films on the alloys containing more than 15 mass % Cr. The value of XCr for each alloy was almost independent of potential in the passivity region below 0.5V. However, in the transpassivity region above 0.6V, it decreased sharply with increasing potential to reach a value close to the mass fraction of Cr in the alloy at 0.70V.