Anodizing of magnesium was studied in solutions containing 3 M KOH + 0.6 M KF + 0.21 M Na3PO4, with and without addition of 0.4 M and 1.1 M of aluminate. At constant applied current, the process occurs in two stages. First, the voltage rises linear ly until an initial breakdown occurs. A thin and compact oxide film is formed at this stage. In the second stage an intense continuous spark is maintained on the metal surface. A thick oxide film is formed at the site of sparking. The formation of the film is accompanied by voltage oscillation and gas evolution. The anodic films formed under continuous breakdown conditions are rough and porous and enriched in aluminum. Anodic films formed in a bath containing 1.1 M aluminate at 15 mA/cm(2) are composed of a single oxide phase MgAl2O4 spinel.