The cylindrical pore structure and barrier layer of anodic films formed on magnesium which are similar to the Keller's model of anodic alumina are determined by direct cross‐sectional observation. The ratios of cell diameter and barrier layer thickness to the forming voltage are notably smaller than those associated with anodic alumina. It is assumed that anodic film growth proceeds mainly by the formation of and (magnesium oxyhydroxide) at the metal/film interface and the dissolution of the film at pore bases.