Ultrathin Fe oxide films of various thicknesses prepared by post-growth oxidation on GaAs(100) surface have been investigated with X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray magnetic circular dichroism (XMCD). The XPS confirms that the surfaces of the oxide are Fe₃O₄ rather than Fe₂O₃. XAS and XMCD measurements indicate the presence of insulating Fe divalent oxide phases (FeO) beneath the surface Fe₃O₄ layer with the sample thickness above 4 nm. This FeO might act as a barrier for the spin injection into the GaAs.