Abstract: An improved anodizing process produces on aluminum and on alloys of aluminum a hard oxide film of greater thickness and better abrasion resistance then can be obtained in the conventional Sanford anodizing process which uses an organic substance sold under the trademark "Sanfran" as an additive to the electrolyte. In the improved process, the DC voltage employed in the conventional process is modulated by a sinusoidal AC voltage of standard industrial frequency and the modulation factor is not permitted to exceed 100% to prevent reversal of the current through the oxide film. The DC voltage is AC modulated during the entire term of the process. In an initial period of 1 to 2 minutes, the DC voltage is raised to about 10 volts and then is raised at a rate of about one half volt per minute to a level within the range of 14 to 19 volts. Upon reaching that level, the DC voltage is held constant for a dwell period of at least five minutes.