The interaction between oxygen and the Fe/Cu(100) interface has been studied as a function of both the iron thickness and the oxygen exposure by means of UPS and XPS spectroscopies. A careful analysis of the Fe 2p spectra has shown that upon exposing Fe thin films (0.5-10 ML) to 10(4) L of oxygen there is always the formation of Fe2O3. Instead, when a copper surface containing 0.5 and 1 ML of iron has been exposed to increasing amounts of oxygen (1-10(4) L), the presence of both Fe2+ and Fe3+ is evident in the middle stage of the oxidation. The UPS spectra have shown a strong decrease of the Cu3d features when the Fe/Cu interface has been exposed to oxygen, even for very thin Fe films (0.5-1 ML). This behaviour can be interpreted in terms of a sizeable decrease of the mean free path of low energy valence band electrons when the iron film is oxidized with respect to that observed in metallic iron films.