The structural and chemical properties of a monolayer FeO(111)/Pt(111) exposed to air, liquid water, and controlled atmospheres of water vapor and water vapor/oxygen mixtures have been studied by a combination of infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning tunneling microscopy experiments together with density functional calculations using ab initio thermodynamics. The FeO(111)/Pt(111) film is inert toward pure water vapor up to mbar pressure. Coadsorption of oxygen and water, however, transforms the film into a hydroxyl terminated trilayer with a (Pt-)O-Fe-OH structural motif. The trilayer film forms spontaneously upon contact of FeO(111)/Pt(111) with air and preserves long-range order even in liquid water. The calculated phase diagram in the relevant range of oxygen and water chemical potentials is in agreement with the experimental results and shows that the oxygen chemical potential is the main driving force for the formation of the trilayer film. Results for Au nucleation on and CO oxidation over the FeO(OH) film are presented and compared to previous studies on nonhydroxylated films.