The result obtained from linear potential sweep indicates that magnesium in 6 M KOH solution behaves like an active-passive metal. The anodic oxidation process of magnesium and the interface structure between magnesium and solution were investigated by alternative current impedance analysis. It is found that when anodized, magnesium experiences active dissolution, passivation and secondary oxidation, In active region, magnesium dissolves actively, which is controlled by charge transfer step. In passive region, magnesium is passivated by the deposited Mg(OH)(2) but its oxidation controlled by charge transfer step still happens. In the secondary oxidation region, magnesium is oxidized directly to MgO, which is mix-controlled by charge transfer step and diffusion of water. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.