Abstract: Five different capacitor grade aluminum foils ranging in purity from 99.97 to 99.999 per cent Al were electrolytically etched in hydrochloric acid under 50Hz A.C. with po-sitive half cycle rectified. The impurities in aluminum were found to influence markedly on the results. The etch morphology of extra pure foil exhibits a deep layer of regularly distributed cubic pits, giving a higher surface area gain.When the impurities in alumin-um increased, the etch pits grew larger and the new pits developed only along the (100) direction at the later stage of etching, forming tappered cubic tunnels and having a lower surface area gain. The results were interpreted by the potential oscillographs and were considered to be related to the ability of passivation of aluminum foil with diffe-rent purity. Because the acidity inner the pits was higher than that in the bulk solu-tion, the etch film formed was thin and transparent, quite differing from the thick pre-cipitated one as in the case of full A.C. etching, which gives a higher pH value owing to the hydrogen evolution during the negative half cycle. Some additives and their effects were also investigated.