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Surface analysis by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry was applied to investigate the origin of the enhanced surface passivity and resistance to a chloride-induced breakdown provided by the protective ultrathin oxide films formed on Cr–Fe–Co–Ni–Mo single-phase fcc multi-principal element alloys. A bilayer structure of the oxide films is observed with the inner barrier layer mostly constituted of Cr(III) oxide and the outer layer enriched in Cr(III) hydroxides and Mo(IV,VI) oxides. The Mo(VI) and Mo(IV) species are mainly located in the outer and inner parts of the outer layer, respectively. Anodic passivation promotes mainly the growth of the inner layer on the alloy of higher Cr bulk content and the outer layer on the alloy of higher Mo bulk content. Passivation also promotes the enrichment of Cr(III) hydroxide and Mo(IV) and Mo(VI) oxides in the outer layer. Depth …