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Observation of tunnels formed on D. C. etched {100} highly ordered aluminum and analysis of the tunnel shape near the tunnel mouth were performed by using the oxide replica method. The tunnel shape near the tunnel mouth was measured as a function of the tunnel depth (li) for the tunnels obtained by the D. C. etching in HCl, NaCl, AlCl3, HCl+AlCl3, NaCl+AlCl3, H2SO4+AlCl3, and HCl+NaCl aqueous solutions. The tunnel width (Wi) tapered exponentially with li, i. e., log (Wi/Wo)=a·li, where Wo is the tunnel width at the tunnel mouth, in all the solutions examined in this study. In the solutions containing H+ ions, the tunnel width increased with the increase in the tunnel depth near the tunnel mouth at depths of 4 to 5μm from the aluminum surface, followed by a decrease of the tunnel width at depths of 5μm or more. The degree of expansion of the tunnel width increased with increasing H+ concentration. This indicated that the increase of the tunnel width with an increasing tunnel depth near the tunnel mouth was closely related to the presence of H+ ions. Corrosion (Al→Al3++3e-, 2H++2e-→H2) of aluminum near the tunnel mouth, in which H+ took part, was suggested as the origin of the tunnel width expansion near the tunnel mouth. In HCl+AlCl3, NaCl+AlCl3, and H2SO4+AlCl3 aqueous solutions, the degree of expansion of tunnel width decreased rapidly compared to the HCl and AlCl3 solutions. Al3+, Cl-, and H+ ion played an important part in determination of the tunnel shape near the tunnel mouth. The rapid decrease of the degree of the tunnel width expansion near the tunnel mouth was considered to be caused by the increase in the concentration of Al3+, Cl-, and H+ ion in the tunnel, followed by the hindrance of the electrolytic dissolution of Al.