Abstract: The influence of temperature, current density, foil purity and the additive on A.C.etching of aluminum electrolytic capacitor foil in hydrochloric acid were studied. It was shown that in order to obtain a high developed surface, it is necessary to have a high quality etch film, in its formation SO₄^2- ion plays an important role. The results were interpreted in terms of the potential response patterns and the film weight on per unit area of real surface, calculated from the weight loss before and after stripping off the film and the coefficient of surface area gain K. The formation of the black powders and smutt-ing of the surface after etching with a certain quantity of electricity may be attributed to the higher pH value developed during the cathodic period, causing the precipitation of volu-minous hydrated aluminum oxide and blocking of the etch pits. Sufficient supply of H⁺ ion is another important condition for high developed surface of foil.