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 positive 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 aluminum 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 different purity. Because the acidity inner the pits was higher than that in the bulk solution, the etch film formed was thin and transparent, quite differing from the thick precipitated 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.