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[Display omitted] • In-situ ambient pressure XPS methodology for electrochemical oxide growth presented. • Quantitative analysis for ambient pressure XPS data further developed. • Enrichment of MoO 3 observed at higher applied potentials. Ambient Pressure X-ray Photoelectron Spectroscopy combined with an electrochemical setup is used to study, in situ , the electrochemical oxide growth on an industrial Ni-Cr-Mo alloy. The native oxide film was characterized in vacuum and in water vapor at 17 mbar, and was found to be 11.4 Å thick and rich in Cr3+. In 0.1 M NaCl electrolyte, anodic growth of the oxide film at potentials up to 700 mV vs Ag/AgCl nearly doubled the thickness of the oxide film. Moreover, a transformation of the oxide composition occurred, as the oxide became enriched in Mo6+ with a chemical fingerprint more like that of pure MoO 3. Both thermodynamics and kinetics of the oxidation of the alloying elements dictate the oxide film growth and composition. Furthermore, we develop the quantitative analysis of oxide composition and thickness to take into account the attenuation through the liquid water and the water vapor atmosphere. Finally, we discuss the differences between ex situ, UHV, in situ , and operando measurements. Our approach is robust, fast, simple, and suitable for systematically probing metal surfaces after aqueous exposure and electrochemical polarization, which promises wide applications for studies of solid–liquid interfaces in corrosion, batteries, fuel cells, and electrocatalysis. [ABSTRACT FROM AUTHOR]