Constant voltage anodizing of binary Mg Al alloys containing 2, 5, 8, and 12% of aluminum was studied. Anodizing was conducted in a bath containing 3 M KOH + 0.6 M KF + 0.21 M Na3PO4 with additions of 0.013, 0.064, 0.13, and 0.4 M of Al(OH)(3). A thin amorphous film was formed during the first stage of anodizing. Prolonged anodizing caused local breakdown and crystallization of the initially formed anodic films. The chemical composition of the newly formed crystalline film was determined. The elements in the film were mainly magnesium, aluminum, and oxygen. Aluminum penetrated into the film both from the electrolyte and from the Mg Al alloy substrate. The phase composition of the anodic films was found to be a mixture of MgO and MgAl2O4.