The initial oxidation of magnesium at oxygen partial pressures between 1.3 x 10(-8) Pa and 1.3 x 10(-5) Pa and at temperatures just above room temperature has been investigated in situ with X-ray photoelectron spectroscopy (XPS) and ellipsometry. Quantitative analysis of the XPS spectra showed that the initially formed oxide has a higher Mg/O ratio (> 1.3) than bulk MgO. Ellipsometry measurements indicated that the band gap values of the oxide layers are considerably smaller than the value expected for bulk MgO (similar to 2.5 eV vs. 7.8 eV). From the XPS and ellipsometry data recorded as a function of oxidation time the oxidation kinetics have been determined. The kinetics has been described quantitatively with a coupled currents model, involving simultaneous transport of electrons and ions through the oxide layer. (c) 2005 Elsevier B.V. All rights reserved.