The electron conduction properties of the passive film on nickel in acid solution were studied by differential capacitance of nickel electrode in the passive condition, and by redox reactions of the Fe2+/Fe3+ and Q/H2Q systems. A large capacitance value of the passive electrode, which is possibly comparable to that of the double layer, can be ascribed to the space charge capacitance in the passive film which is highly nonstoichiometric. The redox reaction currents of these systems may be sustained by tunnelling of charge carriers through the space charge layer in the passive film, if it can be accepted that the ‘tunnel effect’ commences when the electric field strength in the space charge layer is of the order of 106 V/cm. Consequently, the passive film can not be an insulator, at least in acid solutions.