The anodization of aluminum covered with a thin crystalline thermal oxide layer in a solution of ammoniumpentaborate in water requires less charge than the anodization of a specimen not covered with thermal oxide to the same voltage. This behavior is connected with the formation during anodization of crystalline oxide in addition to the amount initially present and with the formation of oxygen bubbles inside the anodic oxide layer. The crystalline oxide can withstand a higher electric field than the amorphous oxide. This property is used in electrolytic capacitors.