Anodizing of magnesium was studied in sodium metasilicate aqueous solutions with and without 5-sulfosalicylic acid. We investigated the experimental conditions for the formation of magnesium anodic oxide films, such as the temperature of the solution, the cell voltage during anodizing, and the concentration of the 5-sulfosalicylic acid. It was found that the anodic oxide film varied at the temperature of 338 K, and that white-colored Mg-Si-O anodic oxide films formed at lower temperatures than 338 K with high voltage in 2.0 mol·dm−3
sodium metasilicate aqueous solution. Addition of 0.25 mol·dm−3
of 5-sulfosalicylic acid to 2.0 mol·dm−3
sodium metasilicate aqueous solution yielded hard and gray-colored Mg-Si-O anodic oxide films containing carbon and sulfur elements. In 5 mass% NaCl solution at 308 K, the gray-colored film sealed in boiling distilled water showed higher corrosion resistance than that of the white-colored film prepared in sodium metasilicate solution.