The impedance response of passive iron is described in terms of a model that takes into account the electrochemical processes taking place at both the metal/oxide and oxide/electrolyte interfaces and the influence of the ionic transport in the passive film. The comparison of the theoretical transfer function with experimental data corresponding to iron passivated in aqueous boric acid-borate buffers leads to the conclusion that the potential perturbing signal does not interfere with the equilibrium of the layer formation reactions (deposition and removal of oxygen ions), the faradaic impedance being related to the iron dissolution through the passive film.