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This paper deals with the analysis of tunnel structures in etched aluminium foil for electrolytic capacitors. The dependence of the capacitance of such foil on anodization voltage and on frequency was measured. Electron microscopy data were also used. The product of series capacitance Cs and anodization voltage Uf shows a characteristic decrease with Uf. This behavior is first analyzed with the model in which it is attributed to the closing of tunnels during anodization. This model, however, leads to irremediable inconsistencies and has to be abandoned. It is proposed instead that the main cause of the decrease of CsUf proportional to Uf is the fact that the oxide layers of neighboring tunnels grow against each other during anodization. Results of anodization-stripping-anodization experiments are contrary to the former and in accordance with the latter model.