Separation of intrinsic and extrinsic intensity contributions to plasmon peaks in X-ray photoelectron spectra of free-electron like metals (Me) such as Be, Na, Mg, Al and semiconductors as Si and Ge, necessary for the accurate determination of the thickness of overlayers in the range of a few nanometers, and their composition, is difficult because of their more or less coincident energies. The intrinsic bulk and surface plasmon contributions to Me 2p spectra can be determined separately from the intensities of the metallic and oxidic main peak as obtained from a series of spectra recorded from the unoxidised metal and the oxidised metal with different oxide-film thicknesses. In the present work, this method was applied to XPS Mg 2p spectra. It was shown that the method is very sensitive to deviations in the measured data, and therefore a careful error analysis is required, which has been developed in this work. Furthermore, an alternative method based on the same theory was proposed. This method yielded values of 0.17 and 0.06 for the intrinsic bulk and surface plasmon contributions to the Mg 2p spectrum relative to the Mg 2p main peak for a detection angle of 45degrees. It was demonstrated that the values obtained for the intrinsic bulk and surface excitation contributions determined according to both methods depend on the oxide-thickness range investigated. This observation indicates that commonly applied simplifying assumptions for the oxidation behaviour of Mg, like a layer-by-layer growth mechanism and/or the development of a homogeneous oxide with bulk MgO properties, as for composition and band gap values, do not hold. The pronounced effect of neglect of the intrinsic plasmon intensity contributions on the thickness values determined for MgO films on Mg was shown. (C) 2003 Elsevier B.V. All rights reserved.