A method for using in situ X-ray absorption near edge spectroscopy to determine the composition of passive, transpassive, and nonreducible thin oxide films is described. The method is demonstrated by determining the composition of the passive films formed in pH 4.5, 0.1 M acetate buffer, on 100 Angstrom thick Fe-yCr alloys (y = 8.5, 15, 19, 23 atom %) at potentials : (i) low in the passive region (-0.3 V vs. mercurous sulfate reference electrode, MSE); (ii) high in the passive region (+0.2 V MSE); (iii) in the transpassive region (+0.8 V MSE); and (iv) in the cathodic region where the air-formed film is not fully reduced (e.g., -1.2 V MSE). The nonreducible film at -1.2 V is entirely a Cr3+ oxide/hydroxide, due to reductive dissolution at -1.2 V of all the Fe that was present in the air-formed film. This nonreducible film persists at anodic potentials. At -0.3 V, the passive film on each alloy is significantly enriched in Cr, present as Cr3+. Fe, if present, exists as both Fe2+ and Fe3+. The film at -0.3 V consists primarily of the nonreducible Cr3+ oxide/hydroxide plus a very small amount of additional oxidized alloy; at this potential most Fe dissolves as Fe2+. At +0.2 V, Fe is present in the passive film only as Fe3+, Cr as Cr3+, and the passive film is less enriched in Cr than at -0.3 V. The composition of the transpassive film at +0.8 V is nearly the same as the composition of the film at +0.2 V,with Fe present at Fe3+, but Cr is present as both Cr3+ and a small amount of Cr6+ (10-25% of the total Cr in the oxide). At +0.2 and +0.8 V, the passive film consists of the nonreducible Cr3+ oxide/hydroxide plus significant amounts of additional oxidized alloy. Formation of the +0.2 and +0.8 V films proceeds with little dissolution of Fe or Cr.