Thin films of two iron oxides (4-6 nm), FeO and Fe3O4, have been prepared on MgO(100) substrates under vacuum by evaporating iron in the presence of oxygen. Structures of the oxides were determined by RHEED transmission patterns and LEED. The Fe(2p) and Fe(3p) XPS and Fe(M2,3VV) Auger spectra were used to characterise the chemical environments of iron in these two oxides. It has been shown that the Wustite formed under these conditions is very unstable and can only be preserved for a very short time as a thin film. In contrast, magnetite is very stable at room temperature even when exposed to the laboratory atmosphere. After heating Fe3O4 at 700 K, magnesium coming from the substrate is detected by AES.