We report a high-resolution core-level spectroscopy study of the (4 x 2)-c(8 x 2) GaAs(100) surface prepared by thermal decapping of surfaces grown by molecular-beam epitaxy. Ga and As 3d core levels are analyzed using very strict parameter conditions, and compared with core levels taken from cleaved GaAs(110). Two surface components are resolved in the Ga spectra while only one surface component is necessary for As. We tentatively assign the two Ga surface components to inequivalent Ga dimers in the unit cell; the As surface component is assigned to the threefold-coordinated second-layer As next to the missing row of Ga dimers. The position of the Fermi level on decapped n-type and p-type substrates is investigated in detail. The characterization of these clean decapped (100) surfaces provides a firm basis for further studies of electrical and chemical properties of metal-semiconductor interfaces.