The stability of cerium oxide (ceria) is a major topic in the field of heterogeneous catalysis. When exposed to a reactive environment or treated in a redox step, ceria is prone to changes of its surface morphology, atomic structure, and composition, with a strong impact onto its catalytic properties. Here, we investigate the stability of Pt(111)-supported ultrathin ceria films upon air exposure and during redox cycles under ultrahigh vacuum (UHV) conditions. Scanning probe microscopy, X-ray photoemission spectroscopy, and low-energy electron diffraction show that upon air exposure, a clean ceria film surface gets contaminated by hydroxyls and carbon-containing species whereas a following annealing in molecular oxygen at around 650 °C removes such contaminants and allows to restore the original surface morphology and structure. When clean films are oxidized in a background oxygen pressure (reduced in UHV) at ∼650 °C, the film coverage increases (decreases). The decrease of the film coverage upon reduction is probably due to a release of cerium atoms, which form an alloy with the platinum substrate that acts as a reservoir for cerium atoms. Due to the alloying, the surface work function (WF) of Pt(111) decreases by ΔϕCePt5/Pt-Pt ≈ −0.20 ± 0.05 eV, as observed by Kelvin probe force microscopy. Upon oxidation, the released cerium is used to form new ceria. With respect to WF changes of the Pt(111) surface by the ceria film, a decrease is found for the oxidized film (ΔϕCeO2/Pt-Pt ≈ −0.55 ± 0.05 eV) as well as for the reduced film (ΔϕrCeria/Pt-Pt ≈ −0.60 ± 0.05 eV).