A detailed investigation of the initial film grown on mechanically polished Mg electrodes has been carried out by ex situ X-ray Photoelectron Spectroscopy (XPS) and in situ Photocurrent Spectroscopy (PCS), allowing to reach a detailed picture of the passive layer structure. The XPS data show that the films formed soon after mechanical treatment and immersion in aqueous electrolyte have a bilayer structure, consisting of an ultra-thin MgO inner layer (similar to 2.5 nm) and a Mg(OH)(2) external layer. The thickness of the Mg(OH)(2) layer is a function of immersion time and solution temperature. After mechanical treatment and immersion in aqueous solution at room temperature, the MgO/Mg(OH)(2) layer in some area of electrodes is so thin to allow an electron photoemission process from the Mg Fermi level to the electrolyte conduction band. Only internal photoemission processes are evidenced for Mg electrodes aged in NaOH at 80 degrees C, due the formation of a thicker Mg(OH)(2) layer. From anodic photocurrent spectra an optical band gap of similar to 4.25 eV has been estimated for Mg(OH)(2), lower with respect to the optical gap of the corresponding anhydrous counterpart. (c) 2007 Elsevier Ltd. All rights reserved.