This paper describes a study of the oxidation of a cerium-5 at. % lanthanum alloy in air at ambient temperature. The alloy was exposed for 42 days, during which it was analysed by X-ray diffraction (XRD) at regular intervals. XRD and metallographic examination indicated linear growth in the thickness of the oxide layer over the duration of this study. Metallographic examination of a transverse section of the oxide layer after 42 days revealed the mean thickness to be 7 mu m. Microstructural examination of the oxide layers also revealed the presence of extensive cracking, which promotes the oxidation process by causing the oxide to be non-protective for the underlying metal. Classification codes: A8160: corrosion, oxidation and surface treatment