The stoichiometry and magnetic properties of reactively sputtered Fe-O/Ag multilayers were studied as a function of the Fe-O layer thickness. The thinnest Fe-O (50 Å) layers were highly disordered. However, when the thickness was increased to more than 100 Å, well-crystallized films comprised of a mixture of FeO and Fe 3O 4 were formed. The relative amount of FeO and Fe 3O 4 was thickness-dependent. Very thick (more than 3500 Å) films were essentially single-phase Fe 3O 4.