Abstract: In order to increase the crystalline oxide content of barrier layer oxide and hence its capacitance, aluminum electrolytic capacitor foil is anodized at an average current density of at most 3 mA/cm.sup.2 of effective surface area in an aqueous electrolyte of C.sub.4 to C.sub.7 unsubstituted aliphatic dicarboxylic acid containing 0 to 0.05 wt. % of citric acid or tartaric acid and preferably at a temperature above 70.degree. C. The anodized foil is subjected to a relaxation and reanodization treatment to stabilize the foil. The anodization at low current density promotes high crystalline oxide content of the barrier layer oxide, and the relaxation and reanodization insure a stable barrier layer oxide of increased capacitance.