Electrochemical etching of hard aluminium foil was studied at 100 degrees C in NaCl solutions without and with Na2SO4 in concentrations up to 1.0 M. Addition of Na2SO4 resulted in an increase in electric capacitance, in refinement of etch configuration and in an increase in tunnel density per unit volume. A decrease in the number of pits from which the tunnels grew also occurred. The capacitance increased with increasing concentration of Na2SO4 up to about 0.35 M, and then decreased. Sulphate ions depressed the formation of pits on the outer oxide-covered surface, but enhanced the growth of the pits and the formation of tunnels from the pits. It is suggested that the retardation of pit nucleation and the acceleration of tunnel growth in the presence of SO42- ions can be explained by a partial replacement of Cl- ions from the oxide and metal surface, respectively. Smaller diameter tunnels may be due to the formation of Al-2(SO4)(3) which can, in part, replace more aggressive AlCl3, and to an easier formation of a passivating film on the tunnel walls owing to their slower dissolution in the presence of Al-2(SO4)(3).